Image decoding method and device using DPB parameter for OLS

ABSTRACT

An image decoding method performed by a decoding device according to the present document comprises the steps of: obtaining image information including an output layer set (OLS) decoded picture buffer (DPB) parameter index for a target OLS and DPB parameter information; deriving DPB parameter information for the target OLS on the basis of the OLS DPB parameter index; performing a picture management process on pictures of a DPB on the basis of the DPB parameter information for the target OLS; configuring a reference picture list for a current picture on the basis of the pictures of the DPB; and performing an inter prediction on a block in the current picture on the basis of the reference picture list.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(e), this application is a continuation ofInternational Application No. PCT/KR2020/019334, with an internationalfiling date of Dec. 29, 2020, which claims the benefit of U.S.Provisional Patent Application No. 62/955,354, filed on Dec. 30, 2019,the contents of which are hereby incorporated by reference herein intheir entirety.

BACKGROUND OF DISCLOSURE Field of the Disclosure

This document relates to image coding technology, and more particularly,to an image decoding method for coding image information including DPBparameters mapped to OLS in an image coding system, and an apparatustherefor.

Related Art

Nowadays, the demand for high-resolution and high-quality images such asa high definition (HD) image and an ultra high definition (UHD) imagehas been increasing in various fields. As the image data becomes higherresolution and higher quality, the transmitted information amount or bitamount increases when compared to the conventional image data.Therefore, when image data is transmitted using a medium such as aconventional wired/wireless broadband line or image data is stored usingan existing storage medium, the transmission cost and the storage costthereof are increased.

Accordingly, there is a need for a highly efficient image compressiontechnique for effectively transmitting, storing, and reproducinginformation of high resolution, high-quality images.

SUMMARY

The present disclosure is to provide a method and apparatus forimproving image coding efficiency.

The present disclosure is also to provide a method and apparatus forderiving a decoded picture buffer (DPB) parameter for an output layerset (OLS).

According to an embodiment of the present disclosure, an image decodingmethod performed by a decoding apparatus is provided. The methodincludes obtaining image information including Decoded Picture Buffer(DPB) parameter information and an Output Layer Set (OLS) DPB parameterindex for a target OLS, deriving DPB parameter information for thetarget OLS based on the OLS DPB parameter index, performing a picturemanagement process for pictures of a DPB based on the DPB parameterinformation for the target OLS, constructing a reference picture listfor a current picture based on the pictures of the DPB, and performinginter prediction for a block in the current picture based on thereference picture list.

According to another embodiment of the present disclosure, a decodingapparatus for performing image decoding is provided. The decodingapparatus includes an entropy decoder configured to obtain imageinformation including Decoded Picture Buffer (DPB) parameter informationand an Output Layer Set (OLS) DPB parameter index for a target OLS, aDPB configured to derive DPB parameter information for the target OLSbased on the OLS DPB parameter index, and perform a picture managementprocess for pictures of a DPB based on the DPB parameter information forthe target OLS, and a predictor configured to construct a referencepicture list for a current picture based on the pictures of the DPB, andperform inter prediction for a block in the current picture based on thereference picture list.

According to still another embodiment of the present disclosure, a videoencoding method which is performed by an encoding apparatus is provided.The method includes generating Decoded Picture Buffer (DPB) parameterinformation of a target Output Layer Set (OLS), performing a picturemanagement process for pictures of a DPB based on the DPB parameterinformation for the target OLS, constructing a reference picture listfor a current picture based on the pictures of the DPB, performing interprediction for a block in the current picture based on the referencepicture list, generating an OLS DPB parameter index for the DPBparameter information of the target OLS, and encoding image informationincluding the OLS DPB parameter index, the DPB parameter information andprediction information for the block.

According to still another embodiment of the present disclosure, a videoencoding apparatus is provided. The encoding apparatus includes anentropy encoder configured to generate Decoded Picture Buffer (DPB)parameter information of a target Output Layer Set (OLS), generate anOLS DPB parameter index for the DPB parameter information of the targetOLS, and encode image information including the OLS DPB parameter index,the DPB parameter information, and the prediction information for theblock in the current picture, a DPB configured to perform a picturemanagement process for pictures of a DPB based on the DPB parameterinformation of the target OLS, and a predictor configured to construct areference picture list for the current picture based on the pictures ofthe DPB, and perform inter prediction for the block in the currentpicture based on the reference picture list.

According to still another embodiment of the present disclosure, thereis provided a computer-readable digital storage medium that stores abitstream including image information, which causes a decoding apparatusto perform an image decoding method. In the computer-readable digitalstorage medium, the image decoding method includes obtaining imageinformation including Decoded Picture Buffer (DPB) parameter informationand an Output Layer Set (OLS) DPB parameter index for a target OLS,deriving DPB parameter information for the target OLS based on the OLSDPB parameter index, performing a picture management process forpictures of a DPB based on the DPB parameter information for the targetOLS, constructing a reference picture list for a current picture basedon the pictures of the DPB, and performing inter prediction for a blockin the current picture based on the reference picture list.

According to this document, DPB parameters for OLS can be signaled, andthrough this, DPB can be updated adaptively to OLS, and overall codingefficiency can be improved.

According to this document, index information indicating DPB parametersfor OLS can be signaled, and through this, DPB parameters can be derivedadaptively to OLS, and the overall coding efficiency can be improved byupdating the DPB for OLS based on the derived DPB parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 briefly illustrates an example of a video/image coding device towhich embodiments of the present disclosure are applicable.

FIG. 2 is a schematic diagram illustrating a configuration of avideo/image encoding apparatus to which the embodiment(s) of the presentdisclosure may be applied.

FIG. 3 is a schematic diagram illustrating a configuration of avideo/image decoding apparatus to which the embodiment(s) of the presentdisclosure may be applied.

FIG. 4 illustrates an example of an inter prediction-based video/imageencoding method.

FIG. 5 illustrates an example of an inter prediction-based video/imagedecoding method.

FIG. 6 schematically shows an inter prediction procedure.

FIG. 7 illustratively represents an encoding process according to anembodiment of the present document.

FIG. 8 illustratively represents a decoding process according to anembodiment of the present document.

FIG. 9 schematically shows an image encoding method by an encodingapparatus according to the present document.

FIG. 10 schematically shows an encoding apparatus for performing animage encoding method according to this document.

FIG. 11 schematically shows an image decoding method by a decodingapparatus according to this document.

FIG. 12 schematically shows a decoding apparatus for performing an imagedecoding method according to this document.

FIG. 13 illustrates a structural diagram of a contents streaming systemto which the present disclosure is applied.

DESCRIPTION OF EMBODIMENTS

The present disclosure may be modified in various forms, and specificembodiments thereof will be described and illustrated in the drawings.However, the embodiments are not intended for limiting the disclosure.The terms used in the following description are used to merely describespecific embodiments but are not intended to limit the disclosure. Anexpression of a singular number includes an expression of the pluralnumber, so long as it is clearly read differently. The terms such as“include” and “have” are intended to indicate that features, numbers,steps, operations, elements, components, or combinations thereof used inthe following description exist and it should be thus understood thatthe possibility of existence or addition of one or more differentfeatures, numbers, steps, operations, elements, components, orcombinations thereof is not excluded.

Meanwhile, elements in the drawings described in the disclosure areindependently drawn for the purpose of convenience for explanation ofdifferent specific functions, and do not mean that the elements areembodied by independent hardware or independent software. For example,two or more elements of the elements may be combined to form a singleelement, or one element may be partitioned into plural elements. Theembodiments in which the elements are combined and/or partitioned belongto the disclosure without departing from the concept of the disclosure.

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings. In addition, likereference numerals are used to indicate like elements throughout thedrawings, and the same descriptions on the like elements will beomitted.

FIG. 1 briefly illustrates an example of a video/image coding device towhich embodiments of the present disclosure are applicable.

Referring to FIG. 1 , a video/image coding system may include a firstdevice (source device) and a second device (receiving device). Thesource device may deliver encoded video/image information or data in theform of a file or streaming to the receiving device via a digitalstorage medium or network.

The source device may include a video source, an encoding apparatus, anda transmitter. The receiving device may include a receiver, a decodingapparatus, and a renderer. The encoding apparatus may be called avideo/image encoding apparatus, and the decoding apparatus may be calleda video/image decoding apparatus. The transmitter may be included in theencoding apparatus. The receiver may be included in the decodingapparatus. The renderer may include a display, and the display may beconfigured as a separate device or an external component.

The video source may acquire video/image through a process of capturing,synthesizing, or generating the video/image. The video source mayinclude a video/image capture device and/or a video/image generatingdevice. The video/image capture device may include, for example, one ormore cameras, video/image archives including previously capturedvideo/images, and the like. The video/image generating device mayinclude, for example, computers, tablets and smartphones, and may(electronically) generate video/images. For example, a virtualvideo/image may be generated through a computer or the like. In thiscase, the video/image capturing process may be replaced by a process ofgenerating related data.

The encoding apparatus may encode input image/image. The encodingapparatus may perform a series of procedures such as prediction,transform, and quantization for compression and coding efficiency. Theencoded data (encoded video/image information) may be output in the formof a bitstream.

The transmitter may transmit the encoded image/image information or dataoutput in the form of a bitstream to the receiver of the receivingdevice through a digital storage medium or a network in the form of afile or streaming. The digital storage medium may include variousstorage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and thelike. The transmitter may include an element for generating a media filethrough a predetermined file format and may include an element fortransmission through a broadcast/communication network. The receiver mayreceive/extract the bitstream and transmit the received bitstream to thedecoding apparatus.

The decoding apparatus may decode the video/image by performing a seriesof procedures such as dequantization, inverse transform, and predictioncorresponding to the operation of the encoding apparatus.

The renderer may render the decoded video/image. The renderedvideo/image may be displayed through the display.

Present disclosure relates to video/image coding. For example, themethods/embodiments disclosed in the present disclosure may be appliedto a method disclosed in the versatile video coding (VVC), the EVC(essential video coding) standard, the AOMedia Video 1 (AV1) standard,the 2nd generation of audio video coding standard (AVS2), or the nextgeneration video/image coding standard (ex. H.267 or H.268, etc.).

Present disclosure presents various embodiments of video/image coding,and the embodiments may be performed in combination with each otherunless otherwise mentioned.

In the present disclosure, video may refer to a series of images overtime. Picture generally refers to a unit representing one image in aspecific time zone, and a subpicture/slice/tile is a unit constitutingpart of a picture in coding. The subpicture/slice/tile may include oneor more coding tree units (CTUs). One picture may consist of one or moresubpictures/slices/tiles. One picture may consist of one or more tilegroups. One tile group may include one or more tiles. A brick mayrepresent a rectangular region of CTU rows within a tile in a picture. Atile may be partitioned into multiple bricks, each of which consistingof one or more CTU rows within the tile. A tile that is not partitionedinto multiple bricks may be also referred to as a brick. A brick scan isa specific sequential ordering of CTUs partitioning a picture in whichthe CTUs are ordered consecutively in CTU raster scan in a brick, brickswithin a tile are ordered consecutively in a raster scan of the bricksof the tile, and tiles in a picture are ordered consecutively in araster scan of the tiles of the picture. In addition, a subpicture mayrepresent a rectangular region of one or more slices within a picture.That is, a subpicture contains one or more slices that collectivelycover a rectangular region of a picture. A tile is a rectangular regionof CTUs within a particular tile column and a particular tile row in apicture. The tile column is a rectangular region of CTUs having a heightequal to the height of the picture and a width specified by syntaxelements in the picture parameter set. The tile row is a rectangularregion of CTUs having a height specified by syntax elements in thepicture parameter set and a width equal to the width of the picture. Atile scan is a specific sequential ordering of CTUs partitioning apicture in which the CTUs are ordered consecutively in CTU raster scanin a tile whereas tiles in a picture are ordered consecutively in araster scan of the tiles of the picture. A slice includes an integernumber of bricks of a picture that may be exclusively contained in asingle NAL unit. A slice may consist of either a number of completetiles or only a consecutive sequence of complete bricks of one tile.Tile groups and slices may be used interchangeably in the presentdisclosure. For example, in the present disclosure, a tile group/tilegroup header may be called a slice/slice header.

A pixel or a pel may mean a smallest unit constituting one picture (orimage). Also, ‘sample’ may be used as a term corresponding to a pixel. Asample may generally represent a pixel or a value of a pixel, and mayrepresent only a pixel/pixel value of a luma component or only apixel/pixel value of a chroma component.

A unit may represent a basic unit of image processing. The unit mayinclude at least one of a specific region of the picture and informationrelated to the region. One unit may include one luma block and twochroma (ex. cb, cr) blocks. The unit may be used interchangeably withterms such as block or area in some cases. In a general case, an M×Nblock may include samples (or sample arrays) or a set (or array) oftransform coefficients of M columns and N rows.

In the present description, “A or B” may mean “only A”, “only B” or“both A and B”. In other words, in the present specification, “A or B”may be interpreted as “A and/or B”. For example, “A, B or C” hereinmeans “only A”, “only B”, “only C”, or “any and any combination of A, Band C”.

A slash (/) or a comma (comma) used in the present description may mean“and/or”. For example, “A/B” may mean “A and/or B”. Accordingly, “A/B”may mean “only A”, “only B”, or “both A and B”. For example, “A, B, C”may mean “A, B, or C”.

In the present description, “at least one of A and B” may mean “only A”,“only B”, or “both A and B”. In addition, in the present description,the expression “at least one of A or B” or “at least one of A and/or B”may be interpreted the same as “at least one of A and B”.

In addition, in the present description, “at least one of A, B and C”means “only A”, “only B”, “only C”, or “any combination of A, B and C”.Also, “at least one of A, B or C” or “at least one of A, B and/or C” maymean “at least one of A, B and C”.

In addition, parentheses used in the present description may mean “forexample”. Specifically, when “prediction (intra prediction)” isindicated, “intra prediction” may be proposed as an example of“prediction”. In other words, “prediction” in the present description isnot limited to “intra prediction”, and “intra prediction” may beproposed as an example of “prediction”. Also, even when “prediction (ie,intra prediction)” is indicated, “intra prediction” may be proposed asan example of “prediction”.

In the present description, technical features that are individuallydescribed within one drawing may be implemented individually or may beimplemented at the same time.

The following drawings were created to explain a specific example of thepresent description. Since the names of specific devices described inthe drawings or the names of specific signals/messages/fields arepresented by way of example, the technical features of the presentdescription are not limited to the specific names used in the followingdrawings.

FIG. 2 is a schematic diagram illustrating a configuration of avideo/image encoding apparatus to which the embodiment(s) of the presentdisclosure may be applied. Hereinafter, the video encoding apparatus mayinclude an image encoding apparatus.

Referring to FIG. 2 , the encoding apparatus 200 includes an imagepartitioner 210, a predictor 220, a residual processor 230, and anentropy encoder 240, an adder 250, a filter 260, and a memory 270. Thepredictor 220 may include an inter predictor 221 and an intra predictor222. The residual processor 230 may include a transformer 232, aquantizer 233, a dequantizer 234, and an inverse transformer 235. Theresidual processor 230 may further include a subtractor 231. The adder250 may be called a reconstructor or a reconstructed block generator.The image partitioner 210, the predictor 220, the residual processor230, the entropy encoder 240, the adder 250, and the filter 260 may beconfigured by at least one hardware component (ex. An encoder chipset orprocessor) according to an embodiment. In addition, the memory 270 mayinclude a decoded picture buffer (DPB) or may be configured by a digitalstorage medium. The hardware component may further include the memory270 as an internal/external component.

The image partitioner 210 may partition an input image (or a picture ora frame) input to the encoding apparatus 200 into one or moreprocessors. For example, the processor may be called a coding unit (CU).In this case, the coding unit may be recursively partitioned accordingto a quad-tree binary-tree ternary-tree (QTBTTT) structure from a codingtree unit (CTU) or a largest coding unit (LCU). For example, one codingunit may be partitioned into a plurality of coding units of a deeperdepth based on a quad tree structure, a binary tree structure, and/or aternary structure. In this case, for example, the quad tree structuremay be applied first and the binary tree structure and/or ternarystructure may be applied later. Alternatively, the binary tree structuremay be applied first. The coding procedure according to the presentdisclosure may be performed based on the final coding unit that is nolonger partitioned. In this case, the largest coding unit may be used asthe final coding unit based on coding efficiency according to imagecharacteristics, or if necessary, the coding unit may be recursivelypartitioned into coding units of deeper depth and a coding unit havingan optimal size may be used as the final coding unit. Here, the codingprocedure may include a procedure of prediction, transform, andreconstruction, which will be described later. As another example, theprocessor may further include a prediction unit (PU) or a transform unit(TU). In this case, the prediction unit and the transform unit may besplit or partitioned from the aforementioned final coding unit. Theprediction unit may be a unit of sample prediction, and the transformunit may be a unit for deriving a transform coefficient and/or a unitfor deriving a residual signal from the transform coefficient.

The unit may be used interchangeably with terms such as block or area insome cases. In a general case, an M×N block may represent a set ofsamples or transform coefficients composed of M columns and N rows. Asample may generally represent a pixel or a value of a pixel, mayrepresent only a pixel/pixel value of a luma component or represent onlya pixel/pixel value of a chroma component. A sample may be used as aterm corresponding to one picture (or image) for a pixel or a pel.

In the encoding apparatus 200, a prediction signal (predicted block,prediction sample array) output from the inter predictor 221 or theintra predictor 222 is subtracted from an input image signal (originalblock, original sample array) to generate a residual signal residualblock, residual sample array), and the generated residual signal istransmitted to the transformer 232. In this case, as shown, a unit forsubtracting a prediction signal (predicted block, prediction samplearray) from the input image signal (original block, original samplearray) in the encoder 200 may be called a subtractor 231. The predictormay perform prediction on a block to be processed (hereinafter, referredto as a current block) and generate a predicted block includingprediction samples for the current block. The predictor may determinewhether intra prediction or inter prediction is applied on a currentblock or CU basis. As described later in the description of eachprediction mode, the predictor may generate various information relatedto prediction, such as prediction mode information, and transmit thegenerated information to the entropy encoder 240. The information on theprediction may be encoded in the entropy encoder 240 and output in theform of a bitstream.

The intra predictor 222 may predict the current block by referring tothe samples in the current picture. The referred samples may be locatedin the neighborhood of the current block or may be located apartaccording to the prediction mode. In the intra prediction, predictionmodes may include a plurality of non-directional modes and a pluralityof directional modes. The non-directional mode may include, for example,a DC mode and a planar mode. The directional mode may include, forexample, 33 directional prediction modes or 65 directional predictionmodes according to the degree of detail of the prediction direction.However, this is merely an example, more or less directional predictionmodes may be used depending on a setting. The intra predictor 222 maydetermine the prediction mode applied to the current block by using aprediction mode applied to a neighboring block.

The inter predictor 221 may derive a predicted block for the currentblock based on a reference block (reference sample array) specified by amotion vector on a reference picture. Here, in order to reduce theamount of motion information transmitted in the inter prediction mode,the motion information may be predicted in units of blocks, sub-blocks,or samples based on correlation of motion information between theneighboring block and the current block. The motion information mayinclude a motion vector and a reference picture index. The motioninformation may further include inter prediction direction (L0prediction, L1 prediction, Bi prediction, etc.) information. In the caseof inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. The referencepicture including the reference block and the reference pictureincluding the temporal neighboring block may be the same or different.The temporal neighboring block may be called a collocated referenceblock, a co-located CU (colCU), and the like, and the reference pictureincluding the temporal neighboring block may be called a collocatedpicture (colPic). For example, the inter predictor 221 may configure amotion information candidate list based on neighboring blocks andgenerate information indicating which candidate is used to derive amotion vector and/or a reference picture index of the current block.Inter prediction may be performed based on various prediction modes. Forexample, in the case of a skip mode and a merge mode, the interpredictor 221 may use motion information of the neighboring block asmotion information of the current block. In the skip mode, unlike themerge mode, the residual signal may not be transmitted. In the case ofthe motion vector prediction (MVP) mode, the motion vector of theneighboring block may be used as a motion vector predictor and themotion vector of the current block may be indicated by signaling amotion vector difference.

The predictor 220 may generate a prediction signal based on variousprediction methods described below. For example, the predictor may notonly apply intra prediction or inter prediction to predict one block butalso simultaneously apply both intra prediction and inter prediction.This may be called combined inter and intra prediction (CIIP). Inaddition, the predictor may be based on an intra block copy (IBC)prediction mode or a palette mode for prediction of a block. The IBCprediction mode or palette mode may be used for content image/videocoding of a game or the like, for example, screen content coding (SCC).The IBC basically performs prediction in the current picture but may beperformed similarly to inter prediction in that a reference block isderived in the current picture. That is, the IBC may use at least one ofthe inter prediction techniques described in the present disclosure. Thepalette mode may be considered as an example of intra coding or intraprediction. When the palette mode is applied, a sample value within apicture may be signaled based on information on the palette table andthe palette index.

The prediction signal generated by the predictor (including the interpredictor 221 and/or the intra predictor 222) may be used to generate areconstructed signal or to generate a residual signal. The transformer232 may generate transform coefficients by applying a transformtechnique to the residual signal. For example, the transform techniquemay include at least one of a discrete cosine transform (DCT), adiscrete sine transform (DST), a karhunen-loéve transform (KLT), agraph-based transform (GBT), or a conditionally non-linear transform(CNT). Here, the GBT means transform obtained from a graph whenrelationship information between pixels is represented by the graph. TheCNT refers to transform generated based on a prediction signal generatedusing all previously reconstructed pixels. In addition, the transformprocess may be applied to square pixel blocks having the same size ormay be applied to blocks having a variable size rather than square.

The quantizer 233 may quantize the transform coefficients and transmitthem to the entropy encoder 240 and the entropy encoder 240 may encodethe quantized signal (information on the quantized transformcoefficients) and output a bitstream. The information on the quantizedtransform coefficients may be referred to as residual information. Thequantizer 233 may rearrange block type quantized transform coefficientsinto a one-dimensional vector form based on a coefficient scanning orderand generate information on the quantized transform coefficients basedon the quantized transform coefficients in the one-dimensional vectorform. Information on transform coefficients may be generated. Theentropy encoder 240 may perform various encoding methods such as, forexample, exponential Golomb, context-adaptive variable length coding(CAVLC), context-adaptive binary arithmetic coding (CABAC), and thelike. The entropy encoder 240 may encode information necessary forvideo/image reconstruction other than quantized transform coefficients(ex. values of syntax elements, etc.) together or separately. Encodedinformation (ex. encoded video/image information) may be transmitted orstored in units of NALs (network abstraction layer) in the form of abitstream. The video/image information may further include informationon various parameter sets such as an adaptation parameter set (APS), apicture parameter set (PPS), a sequence parameter set (SPS), or a videoparameter set (VPS). In addition, the video/image information mayfurther include general constraint information. In the presentdisclosure, information and/or syntax elements transmitted/signaled fromthe encoding apparatus to the decoding apparatus may be included invideo/picture information. The video/image information may be encodedthrough the above-described encoding procedure and included in thebitstream. The bitstream may be transmitted over a network or may bestored in a digital storage medium. The network may include abroadcasting network and/or a communication network, and the digitalstorage medium may include various storage media such as USB, SD, CD,DVD, Blu-ray, HDD, SSD, and the like. A transmitter (not shown)transmitting a signal output from the entropy encoder 240 and/or astorage unit (not shown) storing the signal may be included asinternal/external element of the encoding apparatus 200, andalternatively, the transmitter may be included in the entropy encoder240.

The quantized transform coefficients output from the quantizer 233 maybe used to generate a prediction signal. For example, the residualsignal (residual block or residual samples) may be reconstructed byapplying dequantization and inverse transform to the quantized transformcoefficients through the dequantizer 234 and the inverse transformer235. The adder 250 adds the reconstructed residual signal to theprediction signal output from the inter predictor 221 or the intrapredictor 222 to generate a reconstructed signal (reconstructed picture,reconstructed block, reconstructed sample array). If there is noresidual for the block to be processed, such as a case where the skipmode is applied, the predicted block may be used as the reconstructedblock. The adder 250 may be called a reconstructor or a reconstructedblock generator. The generated reconstructed signal may be used forintra prediction of a next block to be processed in the current pictureand may be used for inter prediction of a next picture through filteringas described below.

Meanwhile, luma mapping with chroma scaling (LMCS) may be applied duringpicture encoding and/or reconstruction.

The filter 260 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter260 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 270, specifically, a DPB of thememory 270. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like. The filter 260 may generate variousinformation related to the filtering and transmit the generatedinformation to the entropy encoder 240 as described later in thedescription of each filtering method. The information related to thefiltering may be encoded by the entropy encoder 240 and output in theform of a bitstream.

The modified reconstructed picture transmitted to the memory 270 may beused as the reference picture in the inter predictor 221. When the interprediction is applied through the encoding apparatus, predictionmismatch between the encoding apparatus 200 and the decoding apparatus300 may be avoided and encoding efficiency may be improved.

The DPB of the memory 270 DPB may store the modified reconstructedpicture for use as a reference picture in the inter predictor 221. Thememory 270 may store the motion information of the block from which themotion information in the current picture is derived (or encoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter predictor 221 and used as the motion information of thespatial neighboring block or the motion information of the temporalneighboring block. The memory 270 may store reconstructed samples ofreconstructed blocks in the current picture and may transfer thereconstructed samples to the intra predictor 222.

FIG. 3 is a schematic diagram illustrating a configuration of avideo/image decoding apparatus to which the embodiment(s) of the presentdisclosure may be applied.

Referring to FIG. 3 , the decoding apparatus 300 may include an entropydecoder 310, a residual processor 320, a predictor 330, an adder 340, afilter 350, a memory 360. The predictor 330 may include an interpredictor 331 and an intra predictor 332. The residual processor 320 mayinclude a dequantizer 321 and an inverse transformer 322. The entropydecoder 310, the residual processor 320, the predictor 330, the adder340, and the filter 350 may be configured by a hardware component (ex. Adecoder chipset or a processor) according to an embodiment. In addition,the memory 360 may include a decoded picture buffer (DPB) or may beconfigured by a digital storage medium. The hardware component mayfurther include the memory 360 as an internal/external component.

When a bitstream including video/image information is input, thedecoding apparatus 300 may reconstruct an image corresponding to aprocess in which the video/image information is processed in theencoding apparatus of FIG. 2 . For example, the decoding apparatus 300may derive units/blocks based on block partition related informationobtained from the bitstream. The decoding apparatus 300 may performdecoding using a processor applied in the encoding apparatus. Thus, theprocessor of decoding may be a coding unit, for example, and the codingunit may be partitioned according to a quad tree structure, binary treestructure and/or ternary tree structure from the coding tree unit or thelargest coding unit. One or more transform units may be derived from thecoding unit. The reconstructed image signal decoded and output throughthe decoding apparatus 300 may be reproduced through a reproducingapparatus.

The decoding apparatus 300 may receive a signal output from the encodingapparatus of FIG. 2 in the form of a bitstream, and the received signalmay be decoded through the entropy decoder 310. For example, the entropydecoder 310 may parse the bitstream to derive information (ex.video/image information) necessary for image reconstruction (or picturereconstruction). The video/image information may further includeinformation on various parameter sets such as an adaptation parameterset (APS), a picture parameter set (PPS), a sequence parameter set(SPS), or a video parameter set (VPS). In addition, the video/imageinformation may further include general constraint information. Thedecoding apparatus may further decode picture based on the informationon the parameter set and/or the general constraint information.Signaled/received information and/or syntax elements described later inthe present disclosure may be decoded may decode the decoding procedureand obtained from the bitstream. For example, the entropy decoder 310decodes the information in the bitstream based on a coding method suchas exponential Golomb coding, CAVLC, or CABAC, and output syntaxelements required for image reconstruction and quantized values oftransform coefficients for residual. More specifically, the CABACentropy decoding method may receive a bin corresponding to each syntaxelement in the bitstream, determine a context model using a decodingtarget syntax element information, decoding information of a decodingtarget block or information of a symbol/bin decoded in a previous stage,and perform an arithmetic decoding on the bin by predicting aprobability of occurrence of a bin according to the determined contextmodel, and generate a symbol corresponding to the value of each syntaxelement. In this case, the CABAC entropy decoding method may update thecontext model by using the information of the decoded symbol/bin for acontext model of a next symbol/bin after determining the context model.The information related to the prediction among the information decodedby the entropy decoder 310 may be provided to the predictor (the interpredictor 332 and the intra predictor 331), and the residual value onwhich the entropy decoding was performed in the entropy decoder 310,that is, the quantized transform coefficients and related parameterinformation, may be input to the residual processor 320. The residualprocessor 320 may derive the residual signal (the residual block, theresidual samples, the residual sample array). In addition, informationon filtering among information decoded by the entropy decoder 310 may beprovided to the filter 350. Meanwhile, a receiver (not shown) forreceiving a signal output from the encoding apparatus may be furtherconfigured as an internal/external element of the decoding apparatus300, or the receiver may be a component of the entropy decoder 310.Meanwhile, the decoding apparatus according to the present disclosuremay be referred to as a video/image/picture decoding apparatus, and thedecoding apparatus may be classified into an information decoder(video/image/picture information decoder) and a sample decoder(video/image/picture sample decoder). The information decoder mayinclude the entropy decoder 310, and the sample decoder may include atleast one of the dequantizer 321, the inverse transformer 322, the adder340, the filter 350, the memory 360, the inter predictor 332, and theintra predictor 331.

The dequantizer 321 may dequantize the quantized transform coefficientsand output the transform coefficients. The dequantizer 321 may rearrangethe quantized transform coefficients in the form of a two-dimensionalblock form. In this case, the rearrangement may be performed based onthe coefficient scanning order performed in the encoding apparatus. Thedequantizer 321 may perform dequantization on the quantized transformcoefficients by using a quantization parameter (ex. quantization stepsize information) and obtain transform coefficients.

The inverse transformer 322 inversely transforms the transformcoefficients to obtain a residual signal (residual block, residualsample array).

The predictor may perform prediction on the current block and generate apredicted block including prediction samples for the current block. Thepredictor may determine whether intra prediction or inter prediction isapplied to the current block based on the information on the predictionoutput from the entropy decoder 310 and may determine a specificintra/inter prediction mode.

The predictor 320 may generate a prediction signal based on variousprediction methods described below. For example, the predictor may notonly apply intra prediction or inter prediction to predict one block butalso simultaneously apply intra prediction and inter prediction. Thismay be called combined inter and intra prediction (CIIP). In addition,the predictor may be based on an intra block copy (IBC) prediction modeor a palette mode for prediction of a block. The IBC prediction mode orpalette mode may be used for content image/video coding of a game or thelike, for example, screen content coding (SCC). The IBC basicallyperforms prediction in the current picture but may be performedsimilarly to inter prediction in that a reference block is derived inthe current picture. That is, the IBC may use at least one of the interprediction techniques described in the present disclosure. The palettemode may be considered as an example of intra coding or intraprediction. When the palette mode is applied, a sample value within apicture may be signaled based on information on the palette table andthe palette index.

The intra predictor 331 may predict the current block by referring tothe samples in the current picture. The referred samples may be locatedin the neighborhood of the current block or may be located apartaccording to the prediction mode. In the intra prediction, predictionmodes may include a plurality of non-directional modes and a pluralityof directional modes. The intra predictor 331 may determine theprediction mode applied to the current block by using a prediction modeapplied to a neighboring block.

The inter predictor 332 may derive a predicted block for the currentblock based on a reference block (reference sample array) specified by amotion vector on a reference picture. In this case, in order to reducethe amount of motion information transmitted in the inter predictionmode, motion information may be predicted in units of blocks,sub-blocks, or samples based on correlation of motion informationbetween the neighboring block and the current block. The motioninformation may include a motion vector and a reference picture index.The motion information may further include inter prediction direction(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. For example, theinter predictor 332 may configure a motion information candidate listbased on neighboring blocks and derive a motion vector of the currentblock and/or a reference picture index based on the received candidateselection information. Inter prediction may be performed based onvarious prediction modes, and the information on the prediction mayinclude information indicating a mode of inter prediction for thecurrent block.

The adder 340 may generate a reconstructed signal (reconstructedpicture, reconstructed block, reconstructed sample array) by adding theobtained residual signal to the prediction signal (predicted block,predicted sample array) output from the predictor (including the interpredictor 332 and/or the intra predictor 331). If there is no residualfor the block to be processed, such as when the skip mode is applied,the predicted block may be used as the reconstructed block.

The adder 340 may be called reconstructor or a reconstructed blockgenerator. The generated reconstructed signal may be used for intraprediction of a next block to be processed in the current picture, maybe output through filtering as described below, or may be used for interprediction of a next picture.

Meanwhile, luma mapping with chroma scaling (LMCS) may be applied in thepicture decoding process.

The filter 350 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter350 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 360, specifically, a DPB of thememory 360. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 360may be used as a reference picture in the inter predictor 332. Thememory 360 may store the motion information of the block from which themotion information in the current picture is derived (or decoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter predictor 260 so as to be utilized as the motion informationof the spatial neighboring block or the motion information of thetemporal neighboring block. The memory 360 may store reconstructedsamples of reconstructed blocks in the current picture and transfer thereconstructed samples to the intra predictor 331.

In the present disclosure, the embodiments described in the filter 260,the inter predictor 221, and the intra predictor 222 of the encodingapparatus 200 may be the same as or respectively applied to correspondto the filter 350, the inter predictor 332, and the intra predictor 331of the decoding apparatus 300. The same may also apply to the unit 332and the intra predictor 331.

In the present disclosure, at least one of quantization/inversequantization and/or transform/inverse transform may be omitted. When thequantization/inverse quantization is omitted, the quantized transformcoefficients may be called transform coefficients. When thetransform/inverse transform is omitted, the transform coefficients maybe called coefficients or residual coefficients, or may still be calledtransform coefficients for uniformity of expression.

In the present disclosure, a quantized transform coefficient and atransform coefficient may be referred to as a transform coefficient anda scaled transform coefficient, respectively. In this case, the residualinformation may include information on transform coefficient(s), and theinformation on the transform coefficient(s) may be signaled throughresidual coding syntax. Transform coefficients may be derived based onthe residual information (or the information on the transformcoefficient(s)), and scaled transform coefficients may be derived byinverse transforming (scaling) on the transform coefficients. Residualsamples may be derived based on the inverse transforming (transforming)on the scaled transform coefficients. This may be applied/expressed inother parts of the present disclosure as well.

Meanwhile, as described above, in performing video coding, prediction isperformed to improve compression efficiency. Through this, a predictedblock including prediction samples for a current block as a block to becoded (i.e., a coding target block) may be generated. Here, thepredicted block includes prediction samples in a spatial domain (orpixel domain). The predicted block is derived in the same manner in anencoding apparatus and a decoding apparatus, and the encoding apparatusmay signal information (residual information) on residual between theoriginal block and the predicted block, rather than an original samplevalue of an original block, to the decoding apparatus, therebyincreasing image coding efficiency. The decoding apparatus may derive aresidual block including residual samples based on the residualinformation, add the residual block and the predicted block to generatereconstructed blocks including reconstructed samples, and generate areconstructed picture including the reconstructed blocks.

The residual information may be generated through a transform andquantization procedure. For example, the encoding apparatus may derive aresidual block between the original block and the predicted block,perform a transform procedure on residual samples (residual samplearray) included in the residual block to derive transform coefficients,perform a quantization procedure on the transform coefficients to derivequantized transform coefficients, and signal related residualinformation to the decoding apparatus (through a bit stream). Here, theresidual information may include value information of the quantizedtransform coefficients, location information, a transform technique, atransform kernel, a quantization parameter, and the like. The decodingapparatus may perform dequantization/inverse transform procedure basedon the residual information and derive residual samples (or residualblocks). The decoding apparatus may generate a reconstructed picturebased on the predicted block and the residual block. Also, for referencefor inter prediction of a picture afterward, the encoding apparatus mayalso dequantize/inverse-transform the quantized transform coefficientsto derive a residual block and generate a reconstructed picture basedthereon.

Intra prediction may refer to prediction that generates predictionsamples for a current block based on reference samples in a picture towhich the current block belongs (hereinafter, referred to as a currentpicture). When the intra prediction is applied to the current block,neighboring reference samples to be used for the intra prediction of thecurrent block may be derived. The neighboring reference samples of thecurrent block may include a sample adjacent to the left boundary of thecurrent block of size nW×nH and a total of 2×nH samples adjacent to thebottom-left of the current block, a sample adjacent to the top boundaryof the current block and a total of 2×nW samples adjacent to thetop-right and a sample adjacent to the top-left of the current block.Alternatively, the neighboring reference samples of the current blockmay include a plurality of columns of top neighboring samples and aplurality of rows of left neighboring samples. In addition, theneighboring reference samples of the current block may include a totalof nH samples adjacent to the right boundary of the current block ofsize nW×nH, a total of nW samples adjacent to the bottom boundary of thecurrent block and a sample adjacent to the bottom-right of the currentblock.

However, some of the neighboring reference samples of the current blockhave not yet been decoded or may not be available. In this case, thedecoder may construct neighboring reference samples to be used forprediction by substituting unavailable samples with available samples.Alternatively, neighboring reference samples to be used for predictionmay be configured through interpolation of available samples.

When the neighboring reference samples are derived, (i) a predictionsample may be derived based on the average or interpolation ofneighboring reference samples of the current block, or (ii) theprediction sample may be derived based on a reference sample existing ina specific (prediction) direction with respect to a prediction sampleamong the neighboring reference samples of the current block. The caseof (i) may be called a non-directional mode or a non-angular mode, andthe case of (ii) may be called a directional mode or an angular mode.

In addition, the prediction sample may be generated throughinterpolation of a first neighboring sample located in the predictiondirection of the intra prediction mode of the current block based on theprediction sample of the current block and a second neighboring samplelocated in a direction opposite to the prediction direction among theneighboring reference samples. The above-described case may be referredto as linear interpolation intra prediction (LIP). In addition, chromaprediction samples may be generated based on the luma samples using alinear model (LM). This case may be called an LM mode or a chromacomponent LM (CCLM) mode.

In addition, a temporary prediction sample of the current block isderived based on the filtered neighboring reference samples, and aprediction sample of the current block may also be derived by weightedsumming the temporary prediction sample and at least one referencesample derived according to the intra prediction mode among the existingneighboring reference samples, that is, unfiltered neighboring referencesamples. The above-described case may be referred to as positiondependent intra prediction (PDPC).

In addition, a reference sample line with the highest predictionaccuracy among neighboring multiple reference sample lines of thecurrent block is selected, and a prediction sample is derived using areference sample located in the prediction direction in the selectedline. In this case, intra prediction encoding may be performed byindicating (signaling) the used reference sample line to the decodingapparatus. The above-described case may be referred to asmulti-reference line intra prediction or MRL-based intra prediction.

In addition, the current block is divided into vertical or horizontalsub-partitions and intra prediction is performed based on the same intraprediction mode, but neighboring reference samples may be derived andused in units of the sub-partitions. That is, in this case, the intraprediction mode for the current block is equally applied to thesub-partitions, but the intra prediction performance may be improved insome cases by deriving and using the neighboring reference samples inunits of the sub-partitions. This prediction method may be calledintra-prediction based on intra sub-partitions (ISP).

The above-described intra prediction methods may be called intraprediction types to be distinguished from the intra prediction mode. Theintra prediction types may be referred to by various terms such as intraprediction technique or additional intra prediction modes. For example,the intra prediction types (or additional intra prediction modes, etc.)may include at least one of the aforementioned LIP, PDPC, MRL, and ISP.A general intra prediction method excluding a specific intra predictiontype such as LIP, PDPC, MRL, and ISP may be referred to as a normalintra prediction type. The normal intra prediction type may be generallyapplied when the above specific intra prediction type is not applied,and prediction may be performed based on the above-described intraprediction mode. Meanwhile, if necessary, post-processing filtering maybe performed on the derived prediction sample.

Specifically, the intra prediction process may include an intraprediction mode/type determination step, neighboring reference samplesderivation step, and an intra prediction mode/type based predictionsample derivation step. In addition, if necessary, a post-filtering stepmay be performed on the derived prediction sample.

When intra prediction is applied, an intra prediction mode applied tothe current block may be determined using an intra prediction mode of aneighboring block. For example, the decoding device may select one ofmost probable mode (MPM) candidates in the MPM list derived based onadditional candidate modes and an intra prediction mode of theneighboring block (eg, the left and/or top neighboring block) of thecurrent block, or select one of the remaining intra prediction modes notincluded in the MPM candidates (and planar mode) based on the remainingintra prediction mode information. The MPM list may be configured toinclude or not include the planner mode as a candidate. For example,when the MPM list includes a planner mode as a candidate, the MPM listmay have 6 candidates, and when the MPM list does not include a plannermode as a candidate, the MPM list may have 5 candidates. When the MPMlist does not include the planar mode as a candidate, a not planar flag(ex. intra_luma_not_planar_flag) representing whether the intraprediction mode of the current block is not the planar mode may besignaled. For example, the MPM flag may be signaled first, and the MPMindex and not planner flag may be signaled when the value of the MPMflag is 1. Also, the MPM index may be signaled when the value of the notplanner flag is 1. Here, the fact that the MPM list is configured not toinclude the planner mode as a candidate is that the planner mode isalways considered as MPM rather than that the planner mode is not MPM,thus, the flag (not planar flag) is signaled first to check whether itis the planar mode.

For example, whether the intra prediction mode applied to the currentblock is among the MPM candidates (and the planar mode) or the remainingmodes may be indicated based on the MPM flag (eg, intra_luma_mpm_flag).The MPM flag with a value of 1 may indicate that the intra predictionmode for the current block is within MPM candidates (and planar mode),and The MPM flag with a value of 0 may indicate that the intraprediction mode for the current block is not within MPM candidates (andplanar mode). The not planar flag (ex. intra_luma_not_planar_flag) witha value of 0 may indicate that the intra prediction mode for the currentblock is a planar mode, and the not planar flag with a value of 1 mayindicate that the intra prediction mode for the current block is not theplanar mode. The MPM index may be signaled in the form of an mpm_idx orintra_luma_mpm_idx syntax element, and the remaining intra predictionmode information may be signaled in the form of arem_intra_luma_pred_mode or intra_luma_mpm_remainder syntax element. Forexample, the remaining intra prediction mode information may indicateone of the remaining intra prediction modes not included in the MPMcandidates (and planar mode) among all intra prediction modes byindexing in the order of prediction mode number. The intra predictionmode may be an intra prediction mode for a luma component (sample).Hereinafter, the intra prediction mode information may include at leastone of the MPM flag (ex. intra_luma_mpm_flag), the not planar flag (ex.intra_luma_not_planar_flag), the MPM index (ex. mpm_idx orintra_luma_mpm_idx), or the remaining intra prediction mode information(rem_intra_luma_luma_mpm_mode or intra_luma_mpminder). In the presentdisclosure, the MPM list may be referred to by various terms such as anMPM candidate list and candModeList. When the MIP is applied to thecurrent block, a separate mpm flag (ex. intra_mip_mpm_flag) for the MIP,an mpm index (ex. intra_mip_mpm_idx), and remaining intra predictionmode information (ex. intra_mip_mpm_remainder) may be signaled, and thenot planar flag may not be signaled.

In other words, in general, when a block partition for an image isperformed, the current block to be coded and a neighboring block havesimilar image characteristics. Therefore, there is a high probabilitythat the current block and the neighboring block have the same orsimilar intra prediction mode. Accordingly, the encoder may use theintra prediction mode of the neighboring block to encode the intraprediction mode of the current block.

For example, the encoding device/the decoding device may construct amost probable modes (MPM) list for the current block. The MPM list maybe referred to as the MPM candidate list. Here, the MPM may refer tomodes used to improve coding efficiency in consideration of thesimilarity between the current block and the neighboring blocks duringintra prediction mode coding. As described above, the MPM list may beconstructed to include the planar mode, or may be constructed to excludethe planar mode. For example, when the MPM list includes the planarmode, the number of candidates in the MPM list may be 6. And, when theMPM list does not include the planar mode, the number of candidates inthe MPM list may be 5.

The encoder/decoder may construct an MPM list including five or sixMPMs.

In order to construct the MPM list, three types of modes, such asdefault intra modes, neighbor intra modes, and derived intra modes, maybe considered.

For the neighbor intra modes, two neighbor blocks, that is, a leftneighbor block and a top neighbor block, may be considered.

As described above, if the MPM list is constructed to not include aplanar mode, the planar mode may be excluded from the list, and thenumber of MPM list candidates may be set to five.

Furthermore, a non-directional mode (or a non-angle mode) among theintra prediction modes may include a DC mode based on an average ofneighbor reference samples of a current block or an interpolation-basedplanar mode.

Meanwhile, when inter prediction is applied, the predictor of theencoding apparatus/decoding apparatus may derive prediction samples byperforming inter prediction in units of blocks. The inter prediction maybe applied when performing the prediction on the current block. That is,the predictor (more specifically, inter predictor) of theencoding/decoding apparatus may derive prediction samples by performingthe inter prediction in units of the block. The inter prediction mayrepresent prediction derived by a method dependent to the data elements(e.g., sample values or motion information) of a picture(s) other thanthe current picture. When the inter prediction is applied to the currentblock, a predicted block (prediction sample array) for the current blockmay be derived based on a reference block (reference sample array)specified by the motion vector on the reference picture indicated by thereference picture index. In this case, in order to reduce an amount ofmotion information transmitted in the inter-prediction mode, the motioninformation of the current block may be predicted in units of a block, asubblock, or a sample based on a correlation of the motion informationbetween the neighboring block and the current block. The motioninformation may include the motion vector and the reference pictureindex. The motion information may further include inter-prediction type(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of applying the inter prediction, the neighboring block may includea spatial neighboring block which is present in the current picture anda temporal neighboring block which is present in the reference picture.A reference picture including the reference block and a referencepicture including the temporal neighboring block may be the same as eachother or different from each other. The temporal neighboring block maybe referred to as a name such as a collocated reference block, acollocated CU (colCU), etc., and the reference picture including thetemporal neighboring block may be referred to as a collocated picture(colPic). For example, a motion information candidate list may beconfigured based on the neighboring blocks of the current block and aflag or index information indicating which candidate is selected (used)may be signaled in order to derive the motion vector and/or referencepicture index of the current block. The inter prediction may beperformed based on various prediction modes and for example, in the caseof a skip mode and a merge mode, the motion information of the currentblock may be the same as the motion information of the selectedneighboring block. In the case of the skip mode, the residual signal maynot be transmitted unlike the merge mode. In the case of a motion vectorprediction (MVP) mode, the motion vector of the selected neighboringblock may be used as a motion vector predictor and a motion vectordifference may be signaled. In this case, the motion vector of thecurrent block may be derived by using a sum of the motion vectorpredictor and the motion vector difference.

The motion information may further include L0 motion information and/orL1 motion information according to the inter-prediction type (L0prediction, L1 prediction, Bi prediction, etc.). A L0-direction motionvector may be referred to as an L0 motion vector or MVL0 and anL1-direction motion vector may be referred to as an L1 motion vector orMVL1. A prediction based on the L0 motion vector may be referred to asan L0 prediction, a prediction based on the L1 motion vector may bereferred to as an L1 prediction, and a prediction based on both the L0motion vector and the L1 motion vector may be referred to as abi-prediction. Here, the L0 motion vector may indicate a motion vectorassociated with a reference picture list L0 and the L1 motion vector mayindicate a motion vector associated with a reference picture list L1.The reference picture list L0 may include pictures prior to the currentpicture in an output order and the reference picture list L1 may includepictures subsequent to the current picture in the output order, as thereference pictures. The prior pictures may be referred to as a forward(reference) picture and the subsequent pictures may be referred to as areverse (reference) picture. The reference picture list L0 may furtherinclude the pictures subsequent to the current picture in the outputorder as the reference pictures. In this case, the prior pictures may befirst indexed in the reference picture list L0 and the subsequentpictures may then be indexed. The reference picture list L1 may furtherinclude the pictures prior to the current picture in the output order asthe reference pictures. In this case, the subsequent pictures may befirst indexed in the reference picture list L1 and the prior picturesmay then be indexed. Here, the output order may correspond to a pictureorder count (POC) order.

A video/image encoding process based on inter prediction mayschematically include, for example, the following.

FIG. 4 illustrates an example of an inter prediction-based video/imageencoding method.

The encoding apparatus performs the inter prediction for the currentblock (S400). The encoding apparatus may derive the inter predictionmode and the motion information of the current block and generate theprediction samples of the current block. Here, an inter prediction modedetermining process, a motion information deriving process, and ageneration process of the prediction samples may be simultaneouslyperformed and any one process may be performed earlier than otherprocess. For example, the inter-prediction unit of the encodingapparatus may include a prediction mode determination unit, a motioninformation derivation unit, and a prediction sample derivation unit,and the prediction mode determination unit may determine the predictionmode for the current block, the motion information derivation unit mayderive the motion information of the current block, and the predictionsample derivation unit may derive the prediction samples of the currentblock. For example, the inter-prediction unit of the encoding apparatusmay search a block similar to the current block in a predetermined area(search area) of reference pictures through motion estimation and derivea reference block in which a difference from the current block isminimum or is equal to or less than a predetermined criterion. Areference picture index indicating a reference picture at which thereference block is positioned may be derived based thereon and a motionvector may be derived based on a difference in location between thereference block and the current block. The encoding apparatus maydetermine a mode applied to the current block among various predictionmodes. The encoding apparatus may compare RD cost for the variousprediction modes and determine an optimal prediction mode for thecurrent block.

For example, when the skip mode or the merge mode is applied to thecurrent block, the encoding apparatus may configure a merging candidatelist to be described below and derive a reference block in which adifference from the current block is minimum or is equal to or less thana predetermined criterion among reference blocks indicated by mergecandidates included in the merging candidate list. In this case, a mergecandidate associated with the derived reference block may be selectedand merge index information indicating the selected merge candidate maybe generated and signaled to the decoding apparatus. The motioninformation of the current block may be derived by using the motioninformation of the selected merge candidate.

As another example, when an (A)MVP mode is applied to the current block,the encoding apparatus may configure an (A)MVP candidate list to bedescribed below and use a motion vector of a selected mvp candidateamong motion vector predictor (mvp) candidates included in the (A)MVPcandidate list as the mvp of the current block. In this case, forexample, the motion vector indicating the reference block derived by themotion estimation may be used as the motion vector of the current blockand an mvp candidate having a motion vector with a smallest differencefrom the motion vector of the current block among the mvp candidates maybecome the selected mvp candidate. A motion vector difference (MVD)which is a difference obtained by subtracting the mvp from the motionvector of the current block may be derived. In this case, theinformation on the MVD may be signaled to the decoding apparatus.Further, when the (A)MVP mode is applied, the value of the referencepicture index may be configured as reference picture index informationand separately signaled to the decoding apparatus.

The encoding apparatus may derive the residual samples based on thepredicted samples (S410). The encoding apparatus may derive the residualsamples by comparing original samples and the prediction samples of thecurrent block.

The encoding apparatus encodes image information including predictioninformation and residual information (S420). The encoding apparatus mayoutput the encoded image information in the form of a bitstream. Theprediction information may include information on prediction modeinformation (e.g., skip flag, merge flag or mode index, etc.) andinformation on motion information as information related to theprediction procedure. The information on the motion information mayinclude candidate selection information (e.g., merge index, mvp flag ormvp index) which is information for deriving the motion vector. Further,the information on the motion information may include the information onthe MVD and/or the reference picture index information. Further, theinformation on the motion information may include information indicatingwhether to apply the L0 prediction, the L1 prediction, or thebi-prediction. The residual information is information on the residualsamples. The residual information may include information on quantizedtransform coefficients for the residual samples.

An output bitstream may be stored in a (digital) storage medium andtransferred to the decoding apparatus or transferred to the decodingapparatus via the network.

Meanwhile, as described above, the encoding apparatus may generate areconstructed picture (including reconstructed samples and reconstructedblocks) based on the reference samples and the residual samples. This isto derive the same prediction result as that performed by the decodingapparatus, and as a result, coding efficiency may be increased.Accordingly, the encoding apparatus may store the reconstruction picture(or reconstruction samples or reconstruction blocks) in the memory andutilize the reconstruction picture as the reference picture. The in-loopfiltering process may be further applied to the reconstruction pictureas described above.

A video/image decoding process based on inter prediction mayschematically include, for example, the following.

FIG. 5 illustrates an example of an inter prediction-based video/imagedecoding method.

Referring to FIG. 5 , the decoding apparatus may perform an operationcorresponding to the operation performed by the encoding apparatus. Thedecoding apparatus may perform the prediction for the current blockbased on received prediction information and derive the predictionsamples.

Specifically, the decoding apparatus may determine the prediction modefor the current block based on the received prediction information(S500). The decoding apparatus may determine which inter prediction modeis applied to the current block based on the prediction mode informationin the prediction information.

For example, it may be determined whether the merge mode or the (A)MVPmode is applied to the current block based on the merge flag.Alternatively, one of various inter prediction mode candidates may beselected based on the mode index. The inter prediction mode candidatesmay include a skip mode, a merge mode, and/or an (A)MVP mode or mayinclude various inter prediction modes to be described below.

The decoding apparatus derives the motion information of the currentblock based on the determined inter prediction mode (S510). For example,when the skip mode or the merge mode is applied to the current block,the decoding apparatus may configure the merge candidate list to bedescribed below and select one merge candidate among the mergecandidates included in the merge candidate list. Here, the selection maybe performed based on the selection information (merge index). Themotion information of the current block may be derived by using themotion information of the selected merge candidate. The motioninformation of the selected merge candidate may be used as the motioninformation of the current block.

As another example, when an (A)MVP mode is applied to the current block,the decoding apparatus may configure an (A)MVP candidate list to bedescribed below and use a motion vector of a selected mvp candidateamong motion vector predictor (mvp) candidates included in the (A)MVPcandidate list as the mvp of the current block. Here, the selection maybe performed based on the selection information (mvp flag or mvp index).In this case, the MVD of the current block may be derived based on theinformation on the MVD, and the motion vector of the current block maybe derived based on the mvp of the current block and the MVD. Further,the reference picture index of the current block may be derived based onthe reference picture index information. The picture indicated by thereference picture index in the reference picture list for the currentblock may be derived as the reference picture referred for the interprediction of the current block.

Meanwhile, as described below, the motion information of the currentblock may be derived without a candidate list configuration and in thiscase, the motion information of the current block may be derivedaccording to a procedure disclosed in the prediction mode. In this case,the candidate list configuration may be omitted.

The decoding apparatus may generate the prediction samples for thecurrent block based on the motion information of the current block(S520). In this case, the reference picture may be derived based on thereference picture index of the current block and the prediction samplesof the current block may be derived by using the samples of thereference block indicated by the motion vector of the current block onthe reference picture. In this case, in some cases, a predicted samplefiltering procedure for all or some of the prediction samples of thecurrent block may be further performed.

For example, the inter-prediction unit of the decoding apparatus mayinclude a prediction mode determination unit, a motion informationderivation unit, and a prediction sample derivation unit, and theprediction mode determination unit may determine the prediction mode forthe current block based on the received prediction mode information, themotion information derivation unit may derive the motion information(the motion vector and/or reference picture index) of the current blockbased on the information on the received motion information, and theprediction sample derivation unit may derive the predicted samples ofthe current block.

The decoding apparatus generates the residual samples for the currentblock based on the received residual information (S530). The decodingapparatus may generate the reconstruction samples for the current blockbased on the prediction samples and the residual samples and generatethe reconstruction picture based on the generated reconstruction samples(S540). Thereafter, the in-loop filtering procedure may be furtherapplied to the reconstruction picture as described above.

FIG. 6 schematically shows an inter prediction procedure.

Referring to FIG. 6 , as described above, the inter prediction processmay include an inter prediction mode determination step, a motioninformation derivation step according to the determined prediction mode,and a prediction processing (prediction sample generation) step based onthe derived motion information. The inter prediction process may beperformed by the encoding apparatus and the decoding apparatus asdescribed above. In this document, a coding device may include theencoding apparatus and/or the decoding apparatus.

Referring to FIG. 6 , the coding apparatus determines an interprediction mode for the current block (S600). Various inter predictionmodes may be used for the prediction of the current block in thepicture. For example, various modes, such as a merge mode, a skip mode,a motion vector prediction (MVP) mode, an affine mode, a subblock mergemode, a merge with MVD (MMVD) mode, and a historical motion vectorprediction (HMVP) mode may be used. A decoder side motion vectorrefinement (DMVR) mode, an adaptive motion vector resolution (AMVR)mode, a bi-prediction with CU-level weight (BCW), a bi-directionaloptical flow (BDOF), and the like may be further used as additionalmodes. The affine mode may also be referred to as an affine motionprediction mode. The MVP mode may also be referred to as an advancedmotion vector prediction (AMVP) mode. In the present document, somemodes and/or motion information candidates derived by some modes mayalso be included in one of motion information-related candidates inother modes. For example, the HMVP candidate may be added to the mergecandidate of the merge/skip modes, or also be added to an mvp candidateof the MVP mode. If the HMVP candidate is used as the motion informationcandidate of the merge mode or the skip mode, the HMVP candidate may bereferred to as the HMVP merge candidate.

The prediction mode information indicating the inter prediction mode ofthe current block may be signaled from the encoding apparatus to thedecoding apparatus. In this case, the prediction mode information may beincluded in the bitstream and received by the decoding apparatus. Theprediction mode information may include index information indicating oneof multiple candidate modes. Alternatively, the inter prediction modemay be indicated through a hierarchical signaling of flag information.In this case, the prediction mode information may include one or moreflags. For example, whether to apply the skip mode may be indicated bysignaling a skip flag, whether to apply the merge mode may be indicatedby signaling a merge flag when the skip mode is not applied, and it isindicated that the MVP mode is applied or a flag for additionaldistinguishing may be further signaled when the merge mode is notapplied. The affine mode may be signaled as an independent mode orsignaled as a dependent mode on the merge mode or the MVP mode. Forexample, the affine mode may include an affine merge mode and an affineMVP mode.

The coding apparatus derives motion information for the current block(S610). Motion information derivation may be derived based on the interprediction mode.

The coding apparatus may perform inter prediction using motioninformation of the current block. The encoding apparatus may deriveoptimal motion information for the current block through a motionestimation procedure. For example, the encoding apparatus may search asimilar reference block having a high correlation in units of afractional pixel within a predetermined search range in the referencepicture by using an original block in an original picture for thecurrent block and derive the motion information through the searchedreference block. The similarity of the block may be derived based on adifference of phase based sample values. For example, the similarity ofthe block may be calculated based on a sum of absolute differences (SAD)between the current block (or a template of the current block) and thereference block (or the template of the reference block). In this case,the motion information may be derived based on a reference block havinga smallest SAD in a search area. The derived motion information may besignaled to the decoding apparatus according to various methods based onthe inter prediction mode.

The coding apparatus performs inter prediction based on motioninformation for the current block (S620). The coding apparatus mayderive prediction sample(s) for the current block based on the motioninformation. A current block including prediction samples may bereferred to as a predicted block.

Meanwhile, the above-described decoded picture buffer (DPB) may beconceptually constructed with a sub-DPB, and the sub-DPB may include apicture storage buffer for storing a decoded picture of one layer. Thepicture storage buffer may include a decoded picture that is marked as“used for reference” or is retained for future output.

In addition, for multilayer bitstreams, the DPB parameter may not beallocated to each Output Layer Set (OLS), but instead may be allocatedto each layer. For example, a maximum of two DPB parameters may beallocated to each layer. one may be allocated when the layer is anoutput layer (i.e., for example, when the layer can be used forreference and future output), and the other one may be allocated whenthe layer is not an output layer, but is used as a reference layer(e.g., when there is no layer switching, and when the layer can be usedonly as a reference of a picture/slice/block of an output layer). Thisis considered simpler when compared to the DPB parameter for themultilayer bitstream of the HEVC layered extension, where each layer ofthe OLS has its own DPB parameter.

For example, the signaling of the DPB parameter may be like the syntaxand semantic below.

TABLE 1 Descriptor video_parameter_set_rbsp( ) {  ...  if(!vps_all_independent_layers_flag )   vps_num_dpb_params ue(v)  if(vps_num_dpb_params > 0 ) {   same_dpb_size_output_or_nonoutput_flag u(1)  if( vps_max_sublayers_minus1 > 0 )   vps_sublayer_dpb_params_present_flag u(1)  }  for( i = 0; i <vps_num_dpb_params: i++ ) {   dpb_size_only_flag[ i ] u(1)   if(vps_max_sublayers_minus1 > 0 && !vps_all_layers_same_num_sub-layers_flag )    dpb_max_temporal_id[ i ] u(3)   dpb_parameters(dpb_size_only_flag[ i ], dpb_max_temporal_id[ i ],    vps_sublayer_dpb_params_present_flag )  }  for( i = 0; i <vps_max_layers_minus1 && vps_num_dpb_params > 1; i++ ) {   if(!vps_independent_layer_flag[ i ] )    layer_output_dpb_params_idx[ i ]ue(v)   if( LayerUsedAsRefLayerFlag[ i ] &&!same_dpb_size_output_or_non- output_flag )   layer_nonoutput_dpb_params_idx[ i ] ue(v)  }  ... }

For example, Table 1 described above may represent a Video Parameter Set(VPS) including syntax elements for a signaled DPB parameter.

Semantics for the syntax elements shown in Table 1 above may be asfollows.

TABLE 2 vps_num_dpb_params specifies the number of dpb_parameters( )syntax strutcures in the VPS. The value of vps_num_dpb_params shall bein the range of 0 to 16, inclusive. When not present, the value ofvps_num_dpb_params is inferred to be equal to 0.same_dpb_size_output_or_nonoutput_flag equal to 1 specifies that thereis no layer_nonoutput_dpb_params_idx[ i ] syntax element present in theVPS. same_dpb_size_output_or_nonoutput_flag equal to 0 specifies thatthere may or may not be layer_nonoutput_dpb_params_idx[ i ] syntaxelements present in the VPS. vps_sublayer_dpb_params_present_flag isused to control the presence of max_dec_pic_buffering_minus1[ ],max_num_reorder_pics[ ], and max_latency_increase_plus1[ ] syntaxelements in the dpb_parameters( ) syntax strucures in the VPS. When notpresent, vps_sub_dpb_params_info_present_flag is inferred to be equal to0. dpb_size_only_flag[ i ] equal to 1 specifies that themax_num_reorder_pics[ ] and max_latency_increase_plus1[ ] syntaxelements are not present in the i-th dpb_parameters( ) syntax strucuresthe VPS. dpb_size_only_flag[ i ] equal to 0 specifies that themax_num_reorder_pics[ ] and max_latency_increase_plus1[ ] syntaxelements may be present in the i-th dpb_parameters( ) syntax strucuresthe VPS. dpb_max_temporal_id[ i ] specifies the TemporalId of thehighest sublayer representation for which the DPB parameters may bepresent in the i-th dpb_parameters( ) syntax strutcure in the VPS. Thevalue of dpb_max_temporal_id[ i ] shall be in the range of 0 tovps_max_sublayers_minus1, inclusive. When vps_max_sublayers_minus1 isequal to 0, the value of dpb_max_temporal_id[ i ] is inferred to beequal to 0. When vps_max_sublayers_minus1 is greater than 0 andvps_all_layers_same_num_sublayers_flag is equal to 1, the value ofdpb_max_temporal_id[ i ] is inferred to be equal tovps_max_sublayers_minus1. layer_output_dpb_params_idx[ i ] specifies theindex, to the list of dpb_parameters( ) syntax structures in the VPS, ofthe dpb_parameters( ) syntax structure that applies to the i-th layerwhen it is an output layer in an OLS. When present, the value oflayer_output_dpb_params_idx[ i ] shall be in the range of 0 tovps_num_dpb_params − 1, inclusive. If vps_independent_layer_flag[ i ] isequal to 1, the dpb_parameters( ) syntax structure that applies to thei-th layer when it is an output layer is the dpb_parameters( ) syntaxstructure present in the SPS referred to by the layer. Otherwise(vps_independent_layer_flag[ i ] is equal to 0), the followingapplies: - When vps_num_dpb_params is equal to 1, the value oflayer_output_dpb_params_idx[ i ] is inferred to be equal to 0. - It is arequirement of bitstream conformance that the value oflayer_output_dpb_params_idx[ i ] shall be such that dpb_size_only_flag[layer_output_dpb_params_idx[ i ] ] is equal to 0.layer_nonoutput_dpb_params_idx[ i ] specifies the index, to the list ofdpb_parameters( ) syntax structures in the VPS, of the dpb_parameters( )syntax structure that applies to the i-th layer when it is a non-outputlayer in an OLS. When present, the value oflayer_nonoutput_dpb_params_idx[ i ] shall be in the range of 0 tovps_num_dpb_params − 1, inclusive. Ifsame_dpb_size_output_or_nonoutput_flag is equal to 1, the followingapplies: - If vps_independent_layer_flag[ i ] is equal to 1, thedpb_parameters( ) syntax structure that applies to the i-th layer whenit is a non-output layer is the dpb_parameters( ) syntax structurepresent in the SPS referred to by the layer. - Otherwise(vps_independent_layer_flag[ i ] is equal to 0), the value oflayer_nonoutput_dpb_params_idx[ i ] is inferred to be equal tolayer_output_dpb_params_idx[ i ]. Otherwise(same_dpb_size_output_or_nonoutput_flag is equal to 0), whenvps_num_dpb_params is equal to 1, the value oflayer_output_dpb_params_idx[ i ] is inferred to be equal to 0.

For example, the syntax element vps_num_dpb_params may represent thenumber of dpb_parameters( ) syntax structures in the VPS. For example,the value of vps_num_dpb_params may be in the range of 0 to 16. Also,when the syntax element vps_num_dpb_params is not present, the value ofthe syntax element vps_num_dpb_params may be inferred to be equal to 0.

Also, for example, the syntax elementsame_dpb_size_output_or_nonoutput_flag may indicate whether the syntaxelement layer_nonoutput_dpb_params_idx[i] may be present in the VPS. Forexample, when the value of the syntax elementsame_dpb_size_output_or_nonoutput_flag is 1, the syntax elementsame_dpb_size_output_or_nonoutput_flag may indicate that syntax elementlayer_nonoutput_dpb_params_idx[i] is not present in the VPS, while, whenthe value of the syntax element same_dpb_size_output_or_nonoutput_flagis 0, the syntax element same_dpb_size_output_or_nonoutput_flag mayindicate that the syntax element layer_nonoutput_dpb_params_idx[i] maybe present in the VPS.

Also, for example, the syntax elementvps_sublayer_dpb_params_present_flag may be used to control the presenceof syntax elements max_dec_pic_buffering_minus1[ ],max_num_reorder_pics[ ], and max_latency_increase_plus1[ ] in thedpb_parameters( ) syntax structure of the VPS. Also, when the syntaxelement vps_sublayer_dpb_params_present_flag is not present, the valueof the syntax element vps_sublayer_dpb_params_present_flag may beinferred to be equal to 0.

Also, for example, the syntax element dpb_size_only_flag[i] may indicatewhether the syntax elements max_num_reorder_pics[ ] andmax_latency_increase_plus1[ ] may be present in the i-th dpb_parameters() syntax structure of the VPS. For example, when the value of the syntaxelement dpb_size_only_flag[i] is 1, the syntax elementdpb_size_only_flag[i] indicates that the syntax elementsmax_num_reorder_pics[ ] and max_latency_increase_plus1[ ] are notpresent in the i-th dpb_parameters( ) syntax structure of the VPS,while, when the value of the syntax element dpb_size_only_flag[i] is 0,the syntax element dpb_size_only_flag[i] may indicate that the syntaxelements max_num_reorder_pics[ ] and max_latency_increase_plus1[ ] maybe present in the i-th dpb_parameters( ) syntax structure of the VPS.

Also, for example, the syntax element dpb_max_temporal_id[i] mayindicate the TemporalId of the highest sublayer representation in whichthe DPB parameter may exist in the i-th dpb_parameters( ) syntaxstructure in the VPS. Also, the value of dpb_max_temporal_id[i] may bein the range of 0 to vps_max_sublayers_minus1. Also, for example, whenthe value of vps_max_sublayers_minus1 is 0, the value ofdpb_max_temporal_id[i] may be inferred to be 0. Also, for example, whenthe value of vps_max_sublayers_minus1 is greater than 0 andvps_all_layers_same_num_sublayers_flag is 1, the value ofdpb_max_temporal_id[i] may be inferred to be equal tovps_max_sublayers_minus1.

Also, for example, the syntax element layer_output_dpb_params_idx[i] mayspecify the index of the dpb_parameters( ) syntax structure applied tothe i-th layer, which is the output layer of the OLS, to the list ofdpb_parameters( ) syntax structures of the VPS. When the syntax elementlayer_output_dpb_params_idx[i] is present, the value of the syntaxelement layer_output_dpb_params_idx[i] may be in the range of 0 tovps_num_dpb_params−1.

For example, when vps_independent_layer_flag[i] is 1, thedpb_parameters( ) syntax structure applied to the i-th layer which isthe output layer may be the dpb_parameters( ) syntax structure presentin the SPS referred to by the layer.

Alternatively, for example, when vps_independent_layer_flag[i] is 0, thefollowing may be applied.

-   -   When vps_num_dpb_params is 1, the value of        layer_output_dpb_params_idx[i] may be inferred to be equal to 0.    -   It may be a requirement of bitstream conformance that the value        of layer_output_dpb_params_idx[i] is such that the value of        dpb_size_only_flag[layer_output_dpb_params_idx[i]] is equal to        0.

Also, for example, the syntax element layer_nonoutput_dpb_params_idx[i]may specify the index of the dpb_parameters( ) syntax structure appliedto the i-th layer, which is a non-output layer of the OLS, to the listof the dpb_parameters( ) syntax structure of the VPS. When the syntaxelement layer_nonoutput_dpb_params_idx[i] is present, the value of thesyntax element layer_nonoutput_dpb_params_idx[i] may be in the range of0 to vps_num_dpb_params−1.

For example, when same_dpb_size_output_or_nonoutput_flag is 1, thefollowing may be applied.

-   -   When vps_independent_layer_flag[i] is 1, the dpb_parameters( )        syntax structure applied to the i-th layer which is a non-output        layer may be the dpb_parameters( ) syntax structure present in        the SPS referred by the layer.    -   When vps_independent_layer_flag[i] is 0, the value of        layer_nonoutput_dpb_params_idx[i] may be inferred to be equal to        layer_output_dpb_params_idx[i].

Alternatively, for example, when same_dpb_size_output_or_nonoutput_flagis 0, and when vps_num_dpb_params is 1, the value oflayer_output_dpb_params_idx[i] may be inferred to be 0.

Meanwhile, for example, the dpb_parameters( ) syntax structure may belike the syntax and semantic below.

TABLE 3 Descriptor dpb_parameters( dpbSizeOnlyFlag, maxSubLayersMinus1,subLayerInfoFlag ) {  for( i = ( subLayerInfoFlag ? 0 :maxSubLayersMinus1 );    i <= maxSubLayersMinus1; i++ ) {  max_dec_pic_buffering_minus1[ i ] ue(v)   if( !dpbSizeOnlyFlag ) {   max_num_reorder_pics[ i ] uc(v)    max_latency_increase_plus1[ i ]ue(v)   }  } }

Referring to Table 3, the dpb_parameters( ) syntax structure may provideinformation on the DPB size for each CLVS of the CVS, the maximumpicture reorder number, and the maximum latency. The dpb_parameters( )syntax structure may be represented as information on DPB parameters orDPB parameter information.

When the dpb_parameters( ) syntax structure is included in the VPS, theOLS to which the dpb_parameters( ) syntax structure is applied may bespecified by the VPS. In addition, when the dpb_parameters( ) syntaxstructure is included in the SPS, the dpb_parameters( ) syntax structuremay be applied to an OLS including only the lowest layer among thelayers making reference to the SPS, wherein the lowest layer may be anindependent layer.

Semantics for the syntax elements shown in Table 3 above may be asfollows.

TABLE 4 max_dec_pic_buffering_minus1[ i ] plus 1 specifies, for each foreach CLVS of the CVS, the maximum required size of the DPB in units ofpicture storage buffers when Htid is equal to i. The value ofmax_dec_pic_buffering_minus1[ i ] shall be in the range of 0 toMaxDpbSize − 1, inclusive, where MaxDpbSize is as specified in clauseA.4.2. When i is greater than 0, max_dec_pic_buffering_minus1[ i ] shallbe greater than or equal to max_dec_pic_buffering_minus1[ i − 1 ]. Whenmax_dec_pic_buffering_minus1[ i ] is not present for i in the range of 0to maxSubLayersMinus1 − 1, inclusive, due to subLayerInfoFlag beingequal to 0, it is inferred to be equal to max_dec_pic_buffering_minus1[maxSubLayersMinus1 ]. max_num_reorder_pics[ i ] specifies, for each CLVSof the CVS, the maximum allowed number of pictures of the CLVS that canprecede any picture in the CLVS in decoding order and follow thatpicture in output order when Htid is equal to i. The value ofmax_num_reorder_pics[ i ] shall be in the range of 0 tomax_dec_pic_buffering_minus1[ i ]. inclusive. When i is greater than 0,max_num_reorder_pics[ i ] shall be greater than or equal tomax_num_reorder_pics[ i − 1 ]. When max_num_reorder_pics[ i ] is notpresent for i in the range of 0 to maxSubLayersMinus1 − 1, inclusive,due to subLayerInfoFlag being equal to 0, it is inferred to be equal tomax_num_reorder_pics[ maxSubLayersMinus1 ]. max_latency_increase_plus1[i ] not equal to 0 is used to compute the value of MaxLatencyPictures[ i], which specifies, for each CLVS of the CVS, the maximum number ofpictures in the CLVS that can precede any picture in the CLVS in outputorder and follow that picture in decoding order when Htid is equal to i.When max_latency_increase_plus1[ i ] is not equal to 0, the value ofMaxLatencyPictures[ i ] is specified as follows:  MaxLatencyPictures[ i] =   max_num_reorder_pics[ i ] + max_latency_increase_plus1[ i ] −1 (7-73) When max_latency_increase_plus1[ i ] is equal to 0, nocorresponding limit is expressed. The value ofmax_latency_increase_plus1[ i ] shall be in the range of 0 to 2³² − 2,inclusive. When max_latency_increase_plus1[ i ] is not present for i inthe range of 0 to maxSubLayersMinus1 − 1, inclusive, due tosubLayerInfoFlag being equal to 0, it is inferred to be equal tomax_latency_increase_plus1[ maxSubLayersMinus1 ].

For example, the syntax element max_dec_pic_buffering_minus1[i] plus 1may specify, for each CLVS of the CVS, the maximum required size of theDPB in units of picture storage buffers when Htid is equal to i. Forexample, max_dec_pic_buffering_minus1[i] may be information on the DPBsize. For example, the value of the syntax elementmax_dec_pic_buffering_minus1[i] may be in the range of 0 toMaxDpbSize−1. Also, for example, when i is greater than 0,max_dec_pic_buffering_minus1[i] may be greater than or equal tomax_dec_pic_buffering_minus1[i−1]. Also, for example, ifmax_dec_pic_buffering_minus1[i] for i in the range of 0 tomaxSubLayersMinus1−1 is not present, due to subLayerInfoFlag being equalto 0, the value of the syntax element max_dec_pic_buffering_minus1[i]may be inferred to be equal to max_dec_pic_buffering_minus1[maxSubLayersMinus1].

Also, for example, the syntax element max_num_reorder_pics[i] mayspecify, for each CLVS of CVS, the maximum allowed number of pictures ofthe CLVS that can precede all pictures of CLVS in decoding order andfollow the corresponding picture in output order when Htid is equal toi. For example, max_num_reorder_pics[i] may be information on themaximum picture reorder number of the DPB. The value ofmax_num_reorder_pics[i] may be in the range of 0 tomax_dec_pic_buffering_minus1[i]. Also, for example, when i is greaterthan 0, max_num_reorder_pics[i] may be greater than or equal tomax_num_reorder_pics[i−1]. Also, for example, if max_num_reorder_pics[i]for i in the range of 0 to maxSubLayersMinus1−1 is not present, due tosubLayerInfoFlag being equal to 0, the syntax elementmax_num_reorder_pics[i] may be inferred to be equal tomax_num_reorder_pics [maxSubLayersMinus1].

Also, for example, a syntax element max_latency_increase_plus1[i] whosevalue is not 0 may be used to calculate the value ofMaxLatencyPictures[i]. The MaxLatencyPictures[i] may specify, for eachCLVS of CVS, the maximum number of pictures of the CLVS that can precedeall pictures of CLVS in output order and follow the correspondingpicture in decoding order when Htid is equal to i. For example,max_latency_increase_plus1[i] may be information on the maximum latencyof the DPB.

For example, when max_latency_increase_plus1[i] is not 0, the value ofMaxLatencyPictures[i] may be derived as following equation.MaxLatencyPictures[i]=max_num_reorder_pics[i]+max_latency_increase_plus1[i]−1  [Equation1]

Meanwhile, for example, if max_latency_increase_plus1[i] is 0, nocorresponding limit may be expressed. The value of themax_latency_increase_plus1[i] may be in the range of 0 to 2³²−2. Also,for example, if max_latency_increase_plus1[i] for i in the range of 0 tomaxSubLayersMinus1−1 is not present, due to subLayerInfoFlag being equalto 0, the syntax element max_latency_increase_plus1[i] may be inferredto be equal to max_latency_increase_plus1[maxSubLayersMinus1].

Meanwhile, the DPB parameter may be used for output and removal of apicture process as shown in the following table.

TABLE 5 The DPB parameter is used in the output and removal of pictureprocess as follows: The output and removal of (decoded) pictures fromthe DPB before the decoding of the current picture (but after parsingthe slice header of the first slice of the current picture) happensinstantaneously when the first DU of the AU containing the currentpicture is removed from the CPB and proceeds as follows: - The decodingprocess for reference picture list construction as specified in clause8.3.2 and decoding process for reference picture marking as specified inclause 8.3.3 are invoked. - If the current picture is a CLVSS picturethat is not picture 0, the following ordered steps are applied: 1. Thevariable NoOutputOfPriorPicsFlag is derived for the decoder under testas follows: - If the value of pic_width_max_in_luma_samples,pic_height_max_in_luma_samples, chroma_format_idc,separate_colour_plane_flag, bit_depth_minus8, ormax_dec_pic_buffering_minus1[ Htid ] derived for any picture of thecurrent AU is different from the value of pic_width_max_in_luma_samples,pic_height_max_in_luma_samples, chroma_format_idc,separate_colour_plane_flag, bit_depth_minus8, ormax_dec_pic_buffering_minus1[ Htid ], respectively, for the precedingpicture in the same CLVS, NoOutputOfPriorPicsFlag may (but should not)be set to 1 by the decoder under test, regardless of the value ofno_output_of_prior_pics_flag. NOTE - Although settingNoOutputOfPriorPicsFlag equal to no_output_of_prior_pics_flag ispreferred under these conditions, the decoder under test is allowed toset NoOutputOfPriorPicsFlag to 1 in this case. - Otherwise,NoOutputOfPriorPicsFlag is set equal to no_output_of_prior_pics_flag. 2.The value of NoOutputOfPriorPicsFlag derived for the decoder under testis applied for the HRD as follows: - If NoOutputOfPriorPicsFlag is equalto 1, all picture storage buffers in the DPB are emptied without outputof the pictures they contain and the DPB fullness is set equal to 0. -Otherwise (NoOutputOfPriorPicsFlag is equal to 0), all picture storagebuffers containing a picture that is marked as “not needed for output”and “unused for reference” are emptied (without output) and allnon-empty picture storage buffers in the DPB are emptied by repeatedlyinvoking the “bumping” process specified in clause C.5.2.4 and the DPBfullness is set equal to 0. - Otherwise (the current picture is not aCLVSS picture), all picture storage buffers containing a picture whichare marked as “not needed for output” and “unused for reference” areemptied (without output). For each picture storage buffer that isemptied, the DPB fullness is decremented by one. When one or more of thefollowing conditions are true, the “bumping” process specified in clauseC.5.2.4 is invoked repeatedly while further decrementing the DPBfullness by one for each additional picture storage buffer that isemptied, until none of the following conditions are true: -  The numberof pictures in the DPB that are marked as “needed for output” is greater than max_num_reorder_pics[ Htid ]. -  max_latency_increase_plus1[ Htid] is not equal to 0 and there is at least one picture in  the DPB thatis marked as “needed for output” for which the associated variable PicLatencyCount is greater than or equal to MaxLatencyPictures[ Htid]. -  The number of pictures in the DPB is greater than or equal to max_dec_pic_buffering_minus1[ Htid ] + 1.

Meanwhile, the DPB parameter signaling design in the conventional VVCstandard may have at least the following problems.

First, the VVC draft text considered the concept of a sub-DPB, but aphysical decoding apparatus can have only one DPB for decodingmultilayer bitstreams. Therefore, the decoding apparatus needs to knowthe DPB size requirement before decoding OLS in a given multilayerbitstream, but conventional VVC draft texts do not clearly disclose howsuch information is made known.

For example, the DPB size required for OLS in the bitstream may not besimply derived from the sub-DPB size of each layer of the OLS. That is,the DPB size required for OLS may not simply be derived as the sum ofmax_dec_pic_buffering_minus1[ ]+1 values of layers in the OLS. Forexample, the sum of max_dec_pic_buffering_minus1[ ]+1 of each layer inthe OLS may be greater than the actual DPB size. For example, in aspecific access unit, since each layer in the DPB may have a differentreference picture list structure, the number of reconstructed picturesof each layer in the DPB may not be the maximum, and therefore, the DPBsize required for the OLS may not be simply derived from the sum ofmax_dec_pic_buffering_minus1[ ]+1 values of the layers in the OLS.

For example, the following table exemplarily shows pictures required foreach sub-DPB to be present for a bitstream with two spatial scalabilitylayers, a group of pictures (GOP) size of 16, and no temporal sublayer.

TABLE 6 Sub-DPB 0 Sub-DPB 1 Total ref pics in RPL #Ref Total #Ref TotalDPB + current # POC Pics Ref pic list pics Pics Ref pic list pics pics 148 3 16, 24, 32 4 3 16, 24, 32 4 8 2 40 3 24, 32, 48 4 3 24, 32, 48 4 83 36 4 24, 32, 40, 48 5 4 24, 32, 40, 48 5 10 4 34 5 24, 32, 36, 40, 486 4 32, 36, 40, 48 5 11 5 33 5 32, 34, 36, 40, 48 6 4 32, 36, 40, 48 511 6 35 5 32, 34, 36, 40, 48 6 4 32, 36, 40, 48 5 11 7 38 5 32, 34, 36,40, 48 6 4 32, 36, 40, 48 5 11 8 37 6 32, 34, 36, 38, 40, 48 7 3 32, 40,48 4 11 9 39 6 32, 34, 36, 38, 40, 48 7 3 32, 40, 48 4 11 10 44 6 32,34, 36, 38, 40, 48 7 3 32, 40, 48 4 11 11 42 5 32, 36, 40, 44, 48 6 432, 40, 44, 48 5 11 12 41 5 32, 36, 40, 44, 48 6 4 32, 40, 44, 48 5 1113 43 5 32, 36, 40, 44, 48 6 4 32, 40, 44, 48 5 11 14 46 4 32, 40, 44,48 5 4 32, 40, 44, 48 5 10 15 45 5 32, 40, 44, 46, 48 6 4 32, 40, 44, 485 11 16 47 5 32, 40, 44, 46, 48 6 4 32, 40, 44, 48 5 11 Max pics 7 Maxpics 5 11

Referring to Table 6, the base layer (i.e., layer 0) may have a morecomplex RPL structure than layer 1, and the size of sub DPB 0 mayinclude more reference pictures than sub DPB 1 in consideration of thepicture size between the two layers. Also, for example, as shown inTable 6, the maximum number of reference pictures of the two layers(i.e., 12) may be greater than the number of actual total pictures ofthe DPB 11).

Second, the bumping process may not be invoked when it is actuallyneeded. When using the above-described example, the number of picturesof sub-DPB 1 does not reach the maximum sub-DPB size after the firstslice header of a picture having a picture order count (POC) 37 of thesecond layer has been decoded, and thus the bumping process may not beinvoked. That is, if the picture having POC 37 is included, the numberof pictures of sub-DPB 1 may be 4, and the maximum number of pictures ofsub-DPB 1 may increase up to 5. However, since the maximum number ofpictures of the DPB has already been reached, the bumping process mustbe invoked at the corresponding time. Such problem may occur becauseonly the DPB parameter of the current layer is checked under thecondition that the bumping process is invoked. Here, for example, thebumping process may refer to a process of deriving pictures necessaryfor output among pictures in the DPB and removing pictures not used asreferences from the DPB.

Accordingly, this document proposes a solution to the above-describedproblem. The proposed embodiments may be applied individually or incombination.

As an example, a method of signaling the DPB parameter mapped to the OLSin addition to signaling the DPB parameter mapped to each layer isproposed.

Also, as an example, there may be provided a method by which the valueof max_dec_pic_buffering_minus1[i] is derived such that it is equal tothe value obtained by subtracting 1 from the sum of values obtained byadding 1 to max_dec_pic_buffering_minus1[i] for all layers in the OLS,when the DPB parameter does not exist since signaling of the DPBparameter mapped to the OLS may be optional. The method proposed in thisembodiment may be performed based on a flag indicating whether a DPBparameter mapped to the OLS is present. For example, when the value ofthe flag is 1, the flag may indicate that the DPB parameter index forall OLSs including at least one or more layers are present, orotherwise, that is, when the value of the flag is 0, the flag mayindicate that a DPB parameter mapped to the OLS (i.e., the DPB parameterindex for the OLS) is not present. Meanwhile, for example, the flag maybe present for each OLS.

Also, as an example, there may be provided a method by which the valueof max_dec_pic_buffering_minus1[hightest temporal sublayer] of each OLSis not greater than the value obtained by subtracting 1 from the sum ofthe value obtained by subtracting 1 from MaxDpbSize and the valuesobtained by adding 1 to max_dec_pic_buffering_minus1 [hightest temporalsublayer] of layers within the OLS.

Also, as an example, there may be proposed a method in which the DPBparameter allocated to the OLS includes only the DPB size.

Also, as an example, there may be proposed a method of updating acondition for invoking the bumping process in consideration of thenumber of pictures in the DPB and the value ofmax_dec_pic_buffering_minus1[i] of the OLS being processed by thedecoding apparatus.

Meanwhile, for example, embodiment(s) may be applied according to thefollowing procedure.

FIG. 7 illustratively represents an encoding process according to anembodiment of the present document.

Referring to FIG. 7 , the encoding apparatus may decode (reconstructed)pictures (S700). The encoding apparatus may update the DPB based on theDPB parameter (S710). For example, the decoded picture may be basicallyinserted into the DPB, and the decoded picture may be used as areference picture for inter prediction. Also, a decoded picture in theDPB may be deleted based on the DPB parameter. Also, the encodingapparatus may encode image information including the DPB parameter(S720). Also, although not shown, the encoding apparatus may furtherdecode the current picture based on the DPB updated after step S710. Inaddition, the decoded current picture may be inserted into the DPB, andthe DPB including the decoded current picture may be further updatedbased on the DPB parameter before decoding the next picture in decodingorder.

FIG. 8 illustratively represents a decoding process according to anembodiment of the present document.

Referring to FIG. 8 , the decoding apparatus may obtain imageinformation including information on the DPB parameter from thebitstream (S800). The decoding apparatus may output a picture decodedfrom the DPB based on the information on the DPB parameter (S805).Meanwhile, when the layer related to the DPB (or the DPB parameter) isnot an output layer but a reference layer, the step S805 may be omitted.

Also, the decoding apparatus may update the DPB based on the informationon the DPB parameter (S810). The decoded picture may be basicallyinserted into the DPB. Then, the DPB may be updated before decoding thecurrent picture. For example, a decoded picture in the DPB may bedeleted based on the information on the DPB parameter. Here, the DPBupdating may be referred to as DPB management.

The information on the DPB parameter may include the information/syntaxelement disclosed in Tables 1 and 3 described above. Additionally, forexample, different DPB parameter(s) may be signaled depending on whetherthe current layer is an output layer or a reference layer, or differentDPB parameter(s) may be signaled depending on whether the DPB (or DPBparameter) is for OLS (mapped to OLS) as in the embodiment proposed inthis document.

Meanwhile, the decoding apparatus may decode the current picture basedon the DPB (S820). For example, the decoding apparatus may decode thecurrent picture based on inter prediction for the block/slice of thecurrent picture using the decoded picture (prior to the current picture)of the DPB as a reference picture.

Meanwhile, although not shown, the encoding apparatus may decode thecurrent picture based on the DPB updated after the above-described stepS710. In addition, the decoded current picture may be inserted into theDPB, and the DPB including the decoded current picture may be furtherupdated based on the DPB parameter before decoding the next picture.

The syntax and DPB management process to which the embodiments proposedin this document are applied will be described below.

As an embodiment, the signaled video parameter set (VPS) syntax may beas follows.

TABLE 7 Descriptor video_parameter_set_rbsp( ) {  ...  if(!vps_all_independent_layers_flag )   vps_num_dpb_params ue(v)  if(vps_num_dpb_params > 0 ) {   same_dpb_size_output_or_nonoutput_flag u(1)  if( vps_max_sublayers_minus1 > 0 )   vps_sublayer_dpb_params_present_flag u(1)  }  for( i = 0; i <vps_num_dpb_params: i++ ) {   dpb_size_only_flag[ i ] u(1)   if(vps_max_sublayers_minus1 > 0 && !vps_all_layers_same_num_sub-layers_flag )    dpb_max_temporal_id[ i ] u(3)   dpb_parameters(dpb_size_only_flag[ i ], dpb_max_temporal_id[ i ],     vps_sublayer_dpb_params_present_flag )  }  for( i = 0; i <vps_max_layers_minus1 && vps_num_dpb_params > 1; i++ ) {   if(!vps_independent_layer_flag[ i ] )    layer_output_dpb_params_idx[ i ]ue(v)   if( LayerUsedAsRefLayerFlag[ i ] &&!same_dpb_size_output_or_non- output_flag )   layer_nonoutput_dpb_params_idx[ i ] ue(v)  } vps_ols_dpb_params_present_flag u(1)  for( i = 0; i < TotalNumOlss &&    vps_ols_dpb_params_present_flag && vps_num_dpb_params > 1; i++ )  if( NumLayersInOls[ i ] > 1 )    ols_dpb_params_idx[ i ] ue(v)  ... }

Referring to Table 7, the VPS may include syntax elementvps_num_dpb_params, same_dpb_size_output_or_nonoutput_flag,vps_sublayer_dpb_params_present_flag, dpb_size_only_flag[i],dpb_max_temporal_id[i], layer_output_dpb_params_idx[i], and/orlayer_nonoutput_dpb_params_idx[i].

In addition, referring to Table 7, the VPS may further include a syntaxelement vps_ols_dpb_params_present_flag and/or ols_dpb_params_idx[i].

For example, the syntax element vps_ols_dpb_params_present_flag mayindicate whether ols_dpb_params_idx[ ] may be present. For example, whenthe value of vps_ols_dpb_params_present_flag is 1,vps_ols_dpb_params_present_flag may indicate that ols_dpb_params_idx[ ]may be present, while, when the value of vps_ols_dpb_params_present_flagis 0, vps_ols_dpb_params_present_flag may indicate thatols_dpb_params_idx[ ] is not present. Meanwhile, whenvps_ols_dpb_params_present_flag is not present, the value ofvps_ols_dpb_params_present_flag may be inferred to be 0.

Also, for example, when i is less than TotalNumOlss, and whenvps_ols_dpb_params_present_flag is 1, and when vps_num_dpb_params isgreater than 1, and if NumLayersInOls[i] is greater than 1, then thesyntax element ols_dpb_params_idx[i] may be signaled. Theols_dpb_params_idx[i] may be represented as vps_ols_dpb_params_idx[i].

For example, when NumLayersInOls[i] is greater than 1, the syntaxelement ols_dpb_params_idx[i] may specify the index of thedpb_parameters( ) syntax structure applied to the i-th OLS, in the listof dpb_parameters( ) syntax structure of the VPS. That is, for example,the syntax element ols_dpb_params_idx[i] may indicate thedpb_parameters( ) syntax structure of the VPS for the target OLS (i.e.,the i-th OLS). When ols_dpb_params_idx[i] is present, the value ofols_dpb_params_idx[i] may be in the range of 0 to vps_num_dpb_params−1.

Also, for example, when NumLayersInOls[i] is equal to 1, thedpb_parameters( ) syntax structure applied to the i-th OLS may bepresent in the SPS referenced by the layer in the i-th OLS.

Meanwhile, according to the present embodiment,OlsMaxDecPicBufferingMinus1[Htid] may be defined as follows.

TABLE 8 In the HRD and conformance specification, define the value ofOlsMaxDecPicBufferingMinus1[ Htid ] as follows: ... For each bitstreamconformance test, the CPB size (number of bits) is CpbSize[ Htid ][ScIdx ] as specified in clause 7.4.6.3, where ScIdx and the HRDparameters are specified above in this clause, and DPB parametersmax_dec_pic_buffering_minus1[ Htid ], max_num_reorder_pics[ Htid ], andMaxLatencyPictures[ Htid ] for each layer are found in or derived fromthe dpb_parameters( ) syntax structure that applies to the layerdepending on whether the layer is an independent layer and whether thelayer is an output layer of the target OLS. For the target OLS, thevalue of OlsMaxDecPicBufferingMinus1[ Htid] is derived as follows: - Ifvps_ols_dpb_params_present_flag is equal to 1,OlsMaxDecPicBufferingMinus1[ Htid] is equal to the value ofmax_dec_pic_buffering_minus1[ Htid ] in the ols_dpb_params_idx[ opOlsIdx]. - Otherwise, OlsMaxDecPicBufferingMinus1[ Htid] is equal to the sumof max_dec_pic_buffering_minus1[ Htid ] + 1 of each layer in the targetOLS minus 1. ...

For example, referring to Table 8, the value ofOlsMaxDecPicBufferingMinus1[Htid] for the target OLS may be derived asfollows.

For example, when the value of vps_ols_dpb_params_present_flag is 1,OlsMaxDecPicBufferingMinus1[Htid] may be derived such that it is equalto the value of max_dec_pic_buffering_minus1[Htid] inols_dpb_params_idx[opOlsIdx].

Also, for example, in other case, that is, when the value ofvps_ols_dpb_params_present_flag is 0, OlsMaxDecPicBufferingMinus1[Htid]may be derived such that it is equal to the value obtained bysubtracting 1 from the sum of max_dec_pic_buffering_minus1[Htid]+1 ofeach layer in the target OLS.

Also, according to the present embodiment, the picture output andremoval process (i.e., the DPB management process) may be defined asfollows.

TABLE 9 The output and removal of pictures from the DPB before thedecoding of the current picture (but after parsing the slice header ofthe first slice of the current picture) happens instantaneously when thefirst DU of the AU containing the current picture is removed from theCPB and proceeds as follows: - The decoding process for referencepicture list construction as specified in clause 8.3.2 and decodingprocess for reference picture marking as specified in clause 8.3.3 areinvoked. - If the current picture is a CLVSS picture that is not picture0, the following ordered steps are applied: 1. The variableNoOutputOfPriorPicsFlag is derived for the decoder under test asfollows: - If the value of pic_width_max_in_luma_samples,pic_height_max_in_luma_samples, chroma_format_idc,separate_colour_plane_flag, bit_depth_minus8, ormax_dec_pic_buffering_minus1[ Htid ] derived for any picture of thecurrent AU is different from the value of pic_width_max_in_luma_samples,pic_height_max_in_luma_samples, chroma_format_idc,separate_colour_plane_flag, bit_depth_minus8, ormax_dec_pic_buffering_minus1[ Htid ], respectively, for the precedingpicture in the same CLVS, NoOutputOfPriorPicsFlag may (but should not)be set to 1 by the decoder under test, regardless of the value ofno_output_of_prior_pics_flag.  NOTE - Although settingNoOutputOfPriorPicsFlag equal to  no_output_of_prior_pics_flag ispreferred under these conditions, the  decoder under test is allowed toset NoOutputOfPriorPicsFlag to 1 in this  case. - Otherwise,NoOutputOfPriorPicsFlag is set equal to no_output_of_prior_pics_flag. 2.The value of NoOutputOfPriorPicsFlag derived for the decoder under testis applied for the HRD as follows: - If NoOutputOfPriorPicsFlag is equalto 1, all picture storage buffers in the DPB are emptied without outputof the pictures they contain and the DPB fullness is set equal to 0. -Otherwise (NoOutputOfPriorPicsFlag is equal to 0), all picture storagebuffers containing a picture that is marked as “not needed for output”and “unused for reference” are emptied (without output) and allnon-empty picture storage buffers in the DPB are emptied by repeatedlyinvoking the “bumping” process specified in clause C.5.2.4 and the DPBfullness is set equal to 0. - Otherwise (the current picture is not aCLVSS picture), all picture storage buffers containing a picture whichare marked as “not needed for output” and “unused for reference” areemptied (without output). For each picture storage buffer that isemptied, the DPB fullness is decremented by one. When one or more of thefollowing conditions are true, the “bumping” process specified in clauseC.5.2.4 is invoked repeatedly while further decrementing the DPBfullness by one for each additional picture storage buffer that isemptied, until none of the following conditions are true: - The numberof pictures in the DPB that are marked as “needed for output” is greaterthan max_num_reorder_pics[ Htid ]. - max_latency_increase_plus1[ Htid ]is not equal to 0 and there is at least one picture in the DPB that ismarked as “needed for output” for which the associated variablePicLatencyCount is greater than or equal to MaxLatencyPictures[ Htid]. - The number of pictures in the sub-DPB is greater than or equal tomax_dec_pic_buffering_minus1[ Htid ] + 1. - The number of pictures inthe DPB is greater than or equal to OlsMaxDecPicBufferingMinus1[ Htid] +1.

For example, referring to Table 9, the number of pictures of the sub DPBmay be greater than or equal to max_dec_pic_buffering_minus1[Htid]+1.Also, for example, the number of pictures in the DPB may be greater thanor equal to OlsMaxDecPicBufferingMinus1[Htid]+1.

Also, according to the present embodiment, the constraint on the maximumpicture of the DPB (i.e., the maximum number of pictures of the DPB) maybe updated as follows. Here, the maximum number of pictures of the DPBmay be represented as the maximum DPB size.

TABLE 10 When the specified level is not level 8.5, the value ofOlsMaxDecPicBufferingMinus1[ Htid ] + 1 shall be less than or equal toMaxDpbSize, which is derived as follows:  if( PicSizeInSamplesY <= (MaxLumaPs >> 2 ) )   MaxDpbSize = Min( 4 * maxDpbPicBuf, 16 )  else if(PicSizeInSamplesY <= ( MaxLumaPs >> 1 ) )   MaxDpbSize = Min( 2 *maxDpbPicBuf, 16 ) (A.1)  else if( PicSizeInSamplesY <= ( ( 3 *MaxLumaPs ) >> 2 ) )   MaxDpbSize = Min( ( 4 * maxDpbPicBuf ) / 3, 16 ) else   MaxDpbSize = maxDpbPicBuf where MaxLumaPs is specified in TableA.1, and maxDpbPicBuf is equal to 8.

For example, referring to Table 10, when the level is not level 8.5, thevalue of OlsMaxDecPicBufferingMinus1[Htid]+1 may be less than or equalto MaxDpbSize.

Alternatively, as an embodiment, the signaled video parameter set (VPS)syntax may be as follows.

TABLE 11 Descriptor video_parameter_set_rbsp( ) {  ···  if(!vps_all_independent_layers_flag )   vps_num_dpb_params ue(v)  if(vps_num_dpb_params > 0 ) {   same_dpb_size_output_or_nonoutput_flag u(1)  if( vps_max_sublayers_minus1 > 0 )   vps_sublayer_dpb_params_present_flag u(1)  }  for( i = 0; i <vps_num_dpb_params; i++ ) {   dpb_size_only_flag[ i ] u(1)   if(vps_max_sublayers_minus1 > 0 && !vps_all_layers_same_num_sub-layers_flag )    dpb_max_temporal_id[ i ] u(3)   dpb_parameters(dpb_size_only_flag[ i ], dpb_max_temporal_id[ i ],     vps_sublayer_dpb_params_present_flag )  }  for( i = 0; i <vps_max_layers_minus1 && vps_num_dpb_params > 1; i++ ) {   if(!vps_independent_layer_flag[ i ] )    layer_output_dpb_params_idx[ i ]ue(v)   if( LayerUsedAsRefLayerFlag[ i ] &&!same_dpb_size_output_or_non- output_flag )   layer_nonoutput_dpb_params_idx[ i ] ue(v)  }  for( i = 0; i <TotalNumOlss && vps_num_dpb_params > 1; i++ )   if( NumLayersInOls[ i] > 1 ) {    vps_ols_dpb_params_present_flag[ i ] u(1)    if(vps_ols_dpb_params_present_flag[ i ] )     ols_dpb_params_idx[ i ] ue(v)  }  ··· }

Referring to Table 11, the VPS may include syntax elementvps_num_dpb_params, same_dpb_size_output_or_nonoutput_flag,vps_sublayer_dpb_params_present_flag, dpb_size_only_flag[i],dpb_max_temporal_id[i], layer_output_dpb_params_idx[i], and/orlayer_nonoutput_dpb_params_idx[i].

In addition, referring to Table 11, the VPS may further include a syntaxelement vps_ols_dpb_params_present_flag and/or ols_dpb_params_idx[i].

For example, when i is less than TotalNumOlss, and whenvps_num_dpb_params is greater than 1, and if NumLayersInOls[i] isgreater than 1, then the syntax element vps_ols_dpb_params_present_flagmay be signaled. Unlike in the embodiment shown in Table 7 above, inwhich vps_ols_dpb_params_present_flag is signaled without a separatecondition, vps_ols_dpb_params_present_flag can be signaled only when iis less than TotalNumOlss and vps_num_dpb_params is greater than 1.

For example, the syntax element vps_ols_dpb_params_present_flag mayindicate whether ols_dpb_params_idx[ ] may be present. For example, whenthe value of vps_ols_dpb_params_present_flag is 1,vps_ols_dpb_params_present_flag may indicate that ols_dpb_params_idx[ ]may be present, while, when the value of vps_ols_dpb_params_present_flagis 0, vps_ols_dpb_params_present_flag may indicate thatols_dpb_params_idx[ ] is not present. Meanwhile, whenvps_ols_dpb_params_present_flag is not present, the value ofvps_ols_dpb_params_present_flag may be inferred to be 0.

Also, for example, when vps_ols_dpb_params_present_flag is 1, a syntaxelement ols_dpb_params_idx[i] may be signaled.

For example, when NumLayersInOls[i] is greater than 1, the syntaxelement ols_dpb_params_idx[i] may specify the index of thedpb_parameters( ) syntax structure applied to the i-th OLS, in the listof dpb_parameters( ) syntax structure of the VPS. That is, for example,the syntax element ols_dpb_params_idx[i] may indicate thedpb_parameters( ) syntax structure of the VPS for the target OLS (i.e.,the i-th OLS). When ols_dpb_params_idx[i] is present, the value ofols_dpb_params_idx[i] may be in the range of 0 to vps_num_dpb_params−1.

FIG. 9 schematically shows an image encoding method by an encodingapparatus according to the present document. The method disclosed inFIG. 9 may be performed by the encoding apparatus disclosed in FIG. 2 .Specifically, for example, S900 and S940 to S950 of FIG. 9 may beperformed by the entropy encoder of the encoding apparatus; S910 of FIG.9 may be performed by the DPB of the encoding apparatus; and S920 toS930 of FIG. 9 may be performed by the predictor of the encodingapparatus.

The encoding apparatus generates decoded picture buffer (DPB) parameterinformation of a target Output Layer Set (OLS) (S900). The encodingapparatus may generate and encode the decoded picture buffer (DPB)parameter information. The image information may include the decodedpicture buffer (DPB) parameter information. For example, the videoparameter set (VPS) syntax may include the DPB parameter information.

For example, the decoded picture buffer (DPB) parameter information mayinclude the DPB parameter information for the target output layer set(OLS). For example, the DPB parameter information for the target OLS mayinclude information on the DPB size for the target OLS, information onthe maximum picture reorder number of the DBP for the target OLS, and/orinformation on the maximum latency of the DBP for the target OLS. Here,the DPB size may indicate the maximum number of pictures that the DPBcan include.

The syntax element of the information on the DPB size for the target OLSmay be the above-described max_dec_pic_buffering_minus1[i], the syntaxelement of the information on the maximum picture reorder number of theDBP for the target OLS may be the above-describedmax_num_reorder_pics[i], and the syntax element of the information onthe maximum latency of the DBP for the target OLS may be theabove-described max_latency_increase_plus1[i].

The encoding apparatus performs a picture management process forpictures of a DPB based on the DPB parameter information of the targetOLS (S910). The encoding apparatus may perform a picture managementprocess for pictures of a DPB based on the DPB parameter information forthe target OLS. The encoding apparatus may update the DPB based on theDPB parameter information. For example, the encoding apparatus mayperform a picture management process for (decoded) pictures of the DPBbased on the DPB parameter information. For example, the encodingapparatus may add a decoded picture to the DPB, or may remove a decodedpicture in the DPB. For example, the decoded picture in the DPB may beused as a reference picture of inter prediction for the current picture,or the decoded picture in the DPB may be used as an output picture. Thedecoded picture may refer to a picture decoded before the currentpicture in decoding order in the target OLS.

Meanwhile, for example, the encoding apparatus may determine whether abumping process for the pictures in the DPB is performed based oninformation on the DPB size for the target OLS and the number ofpictures in the DPB, and may perform the bumping process for the picturein the DPB based on the determination result. For example, when thenumber of pictures in the DPB is greater than or equal to the valuederived based on the information on the DPB size, the bumping processmay be performed, while, when the number of pictures in the DPB is lessthan the value derived based on the information on the DPB size, thebumping process may not be performed. Here, for example, the valuederived based on the information on the DPB size may be a value obtainedby adding 1 to the value of the information on the DPB size.

The encoding apparatus constructs a reference picture list for thecurrent picture based on the pictures of the DPB (S920). The encodingapparatus may construct a reference picture list for the current picturebased on the pictures of the DPB. That is, the encoding apparatus mayconstruct a reference picture list for the current picture based on theupdated pictures of the DPB. For example, the encoding apparatus mayconstruct a reference picture list based on decoded pictures of a DPBused for inter prediction of blocks in the current picture. For example,the reference picture list for the current picture may include a pictureused for inter prediction with respect to a block of the current pictureamong pictures of the DPB.

The encoding apparatus performs inter prediction on the block in thecurrent picture based on the reference picture list (S930). The encodingapparatus may perform inter prediction on the block in the currentpicture based on the reference picture list. The encoding apparatus maydecode the current picture of the target OLS. The encoding apparatus mayderive a reference picture for a block in the current picture from amongreference pictures of the reference picture list for the block, and mayperform inter prediction on the block based on the reference picture.The encoding apparatus may derive a prediction sample by performinginter prediction on the block based on the derived reference picture andmotion information on the block. The encoding apparatus may generate areconstructed sample and/or a reconstructed picture for the block of thecurrent picture based on the prediction sample. Meanwhile, for example,the encoding apparatus may derive a residual sample for a block in thecurrent picture, and may generate a reconstructed sample and/or areconstructed picture through addition of the prediction sample and theresidual sample.

The encoding apparatus generates an OLS DPB parameter index for the DPBparameter information of the target OLS (S940). The encoding apparatusmay generate and encode an OLS DPB parameter index for the DPB parameterinformation of the target OLS. The image information may include an OLSDPB parameter index for DPB parameter information of the target OLS. Forexample, the VPS syntax may include the OLS DPB parameter index.

For example, the OLS DPB parameter index for the target OLS may indicateDPB parameter information for the target OLS. For example, the OLS DPBparameter index for the target OLS may indicate DPB parameterinformation for the target OLS in the DPB parameter information. Thesyntax element of the OLS DPB parameter index may be the above-describedvps_ols_dpb_params_idx[i] or ols_dpb_params_idx[i].

Meanwhile, for example, the encoding apparatus may generate and encodean OLS DPB parameter flag for whether the DPB parameter information forthe target OLS is present. For example, the image information mayinclude the OLS DPB parameter flag. Additionally, for example, the VPSsyntax may include the OLS DPB parameter flag. For example, the OLS DPBparameter flag may indicate whether the DPB parameter information forthe target OLS is present. For example, when the value of the OLS DPBparameter flag is 1, the OLS DPB parameter flag may indicate that theDPB parameter information for the target OLS may be present, while, whenthe value of the OLS DPB parameter flag is 0, the OLS DPB parameter flagmay indicate that the DPB parameter information for the target OLS isnot present. Also, for example, the OLS DPB parameter index may begenerated and encoded based on the OLS DPB parameter flag. For example,when the value of the OLS DPB parameter flag is 1, the OLS DPB parameterindex may be generated/encoded/signaled, while, when the value of theOLS DPB parameter flag is 0, the OLS DPB parameter index may not begenerated/encoded/signaled. The syntax element of the OLS DPB parameterflag may be the above-described vps_ols_dpb_params_present_flag.

The encoding apparatus encodes image information including the OLS DPBparameter index, the DPB parameter information and predictioninformation for the block (S950). The encoding apparatus may encodeimage information including the OLS DPB parameter index, the DPBparameter information, and the prediction information for the block. Theimage information may include the OLS DPB parameter index, the DPBparameter information, and the prediction information for the block.Also, the image information may include the above-described OLS DPBparameter flag.

For example, the encoding apparatus may generate and encode predictioninformation for a block of the current picture. In this case, variousprediction methods disclosed in the present document, such as interprediction or intra prediction, may be applied. For example, theencoding apparatus may determine whether to perform inter prediction orintra prediction on the block, and may determine a specific interprediction mode or a specific intra prediction mode based on RD cost.According to the determined mode, the encoding apparatus may deriveprediction samples for the current chroma block. The predictioninformation may include prediction mode information for the currentchroma block. The image information may include the predictioninformation. Also, the encoding apparatus may generate and encode areference picture index indicating a reference picture for the block.For example, the prediction information may include the referencepicture index. Also, the encoding apparatus may derive motioninformation for the block, and may generate and encode information onthe motion information. For example, the prediction information mayinclude information related to the motion information.

Meanwhile, for example, the encoding apparatus may encode residualinformation for the block of the picture. For example, the encodingapparatus may derive the residual sample through the subtraction of theoriginal sample and the prediction sample for the block.

Thereafter, for example, the encoding apparatus may derive a quantizedresidual sample by quantizing the residual sample, and may derive atransform coefficient based on the quantized residual sample, and maygenerate and encode the residual information based on the transformcoefficient. Alternatively, for example, the encoding apparatus mayderive a quantized residual sample by quantizing the residual sample,and may derive a transform coefficient by transforming the quantizedresidual sample, and may generate and encode the residual informationbased on the transform coefficient. The image information may includethe residual information. Additionally, for example, the encodingapparatus may encode image information, and output the encoded imageinformation in the form of a bitstream.

The encoding apparatus may generate reconstructed samples and/or areconstructed picture through addition of the prediction samples and theresidual samples. Thereafter, as described above, the in-loop filteringprocess such as an ALF process, SAO and/or deblocking filtering may beapplied as needed to the reconstructed samples in order to improvesubjective/objective video quality.

Meanwhile, the bitstream including the image information may betransmitted to the decoding apparatus through a network or a (digital)storage medium. Here, the network may include a broadcast network, acommunication network and/or the like, and the digital storage mediummay include various storage media, such as a universal serial bus (USB),secure digital (SD), a compact disk (CD), a digital video disk (DVD),Blu-ray, a hard disk drive (HDD), a solid state drive (SSD), and thelike.

FIG. 10 schematically shows an encoding apparatus for performing animage encoding method according to this document. The method disclosedin FIG. 9 may be performed by the encoding apparatus disclosed in FIG.10 . Specifically, for example, the entropy encoder of the encodingapparatus of FIG. 10 may perform S900 and S940 to S950; the DPB of theencoding apparatus of FIG. 10 may perform S910; and the predictor of theencoding apparatus of FIG. 10 may perform S920 to S930.

FIG. 11 schematically shows an image decoding method by a decodingapparatus according to this document. The method disclosed in FIG. 11may be performed by the decoding apparatus disclosed in FIG. 3 .Specifically, for example, S1100 of FIG. 11 may be performed by theentropy decoder of the decoding apparatus; S1110 to S1120 of FIG. 11 maybe performed by the DPB of the decoding apparatus; and S1130 to S1140 ofFIG. 11 may be performed by the predictor of the decoding apparatus.

The decoding apparatus obtains image information including decodedpicture buffer (DPB) parameter information and an output layer set (OLS)DPB parameter index for a target OLS (S1100). The decoding apparatus mayobtain image information including decoded picture buffer (DPB)parameter information and an output layer set (OLS) DPB parameter indexfor a target OLS.

For example, the decoding apparatus may obtain a video parameter set(VPS) syntax from the bitstream. The image information may include theVPS syntax. The image information may be received in a bitstream. TheVPS syntax may include the DPB parameter information and the OLS DPBparameter index for the target OLS. That is, for example, the decodingapparatus may obtain the DPB parameter information and the OLS DPBparameter index for the target OLS with the VPS syntax.

For example, the OLS DPB parameter index for the target OLS may indicateDPB parameter information for the target OLS. For example, the DPBparameter information may include DPB parameter information for thetarget OLS, and the OLS DPB parameter index for the target OLS mayindicate DPB parameter information for the target OLS in the DPBparameter information. The syntax element of the OLS DPB parameter indexmay be the above-described vps_ols_dpb_params_idx[i] orols_dpb_params_idx[i].

Meanwhile, for example, the decoding apparatus may obtain an OLS DPBparameter flag for whether the DPB parameter information for the targetOLS is present. For example, the image information may include the OLSDPB parameter flag. Additionally, for example, the VPS syntax mayinclude the OLS DPB parameter flag. For example, the OLS DPB parameterflag may indicate whether the DPB parameter information for the targetOLS is present. For example, when the value of the OLS DPB parameterflag is 1, the OLS DPB parameter flag may indicate that the DPBparameter information for the target OLS may be present, while, when thevalue of the OLS DPB parameter flag is 0, the OLS DPB parameter flag mayindicate that the DPB parameter information for the target OLS is notpresent. Also, for example, the OLS DPB parameter index may be obtainedbased on the OLS DPB parameter flag. For example, when the value of theOLS DPB parameter flag is 1, the OLS DPB parameter index may besignaled/obtained, while, when the value of the OLS DPB parameter flagis 0, the OLS DPB parameter index may not be signaled/obtained. Thesyntax element of the OLS DPB parameter flag may be the above-describedvps_ols_dpb_params_present_flag.

The decoding apparatus derives DPB parameter information for the targetOLS based on the OLS DPB parameter index (S1110). The decoding apparatusmay derive DPB parameter information for the target OLS based on the OLSDPB parameter index. For example, the decoding apparatus may derive DPBparameter information for the target OLS indicated by the OLS DPBparameter index. The DPB parameter information may include the DPBparameter information for the target OLS.

For example, the DPB parameter information for the target OLS mayinclude information on the DPB size for the target OLS, information onthe maximum picture reorder number of the DBP for the target OLS, and/orinformation on the maximum latency of the DBP for the target OLS. Here,the DPB size may indicate the maximum number of pictures that the DPBcan include.

The syntax element of the information on the DPB size for the target OLSmay be the above-described max_dec_pic_buffering_minus1[i], the syntaxelement of the information on the maximum picture reorder number of theDBP for the target OLS may be the above-describedmax_num_reorder_pics[i], and the syntax element of the information onthe maximum latency of the DBP for the target OLS may be theabove-described max_latency_increase_plus1[i].

The decoding apparatus performs a picture management process forpictures of a DPB based on the DPB parameter information for the targetOLS (S1120). The decoding apparatus may perform a picture managementprocess for pictures of a DPB based on the DPB parameter information forthe target OLS. The decoding apparatus may update the DPB based on theDPB parameter information. For example, the decoding apparatus mayperform a picture management process for (decoded) pictures of the DPBbased on the DPB parameter information. For example, the decodingapparatus may add a decoded picture to the DPB, or may remove a decodedpicture in the DPB. For example, the decoded picture in the DPB may beused as a reference picture of inter prediction for the current picture,or the decoded picture in the DPB may be used as an output picture. Thedecoded picture may refer to a picture decoded before the currentpicture in decoding order in the target OLS.

Meanwhile, for example, the decoding apparatus may determine whether abumping process for the pictures in the DPB is performed based on thenumber of pictures in the DPB and information on the DPB size for thetarget OLS, and may perform the bumping process for the picture in theDPB based on the determination result. For example, when the number ofpictures in the DPB is greater than or equal to the value derived basedon the information on the DPB size, the bumping process may beperformed, while, when the number of pictures in the DPB is less thanthe value derived based on the information on the DPB size, the bumpingprocess may not be performed. Here, for example, the value derived basedon the information on the DPB size may be a value obtained by adding 1to the value of the information on the DPB size.

The decoding apparatus constructs a reference picture list for a currentpicture based on the pictures of the DPB (S1130). The decoding apparatusmay construct a reference picture list for the current picture based onthe pictures of the DPB. That is, the decoding apparatus may construct areference picture list for the current picture based on the updatedpictures of the DPB. For example, the decoding apparatus may construct areference picture list based on decoded pictures of a DPB used for interprediction of blocks in the current picture. For example, the referencepicture list for the current picture may include a picture used forinter prediction with respect to a block of the current picture amongpictures of the DPB.

The decoding apparatus performs inter prediction for a block in thecurrent picture based on the reference picture list (S1140). Thedecoding apparatus may perform inter prediction on the block in thecurrent picture based on the reference picture list. The decodingapparatus may perform inter prediction on the block based on a referencepicture in the reference picture list for the block. For example, thedecoding apparatus may obtain prediction information for the block. Theimage information may include the prediction information. The predictioninformation may include a reference picture index indicating a referencepicture for the block. The decoding apparatus may derive a referencepicture in the reference picture list for the block based on thereference picture index, and may derive a prediction sample byperforming inter prediction on the block based on the motion informationon the block and the reference picture. The decoding apparatus maygenerate a reconstructed sample and/or a reconstructed picture for theblock of the current picture based on the prediction sample. Meanwhile,for example, the decoding apparatus may derive a residual sample for ablock in the current picture based on residual information receivedthrough a bitstream, and may generate a reconstructed picture and/or areconstructed sample through addition of the predicted sample and theresidual sample.

Thereafter, as described above, the in-loop filtering process such as anALF process, SAO and/or deblocking filtering may be applied as needed tothe reconstructed samples in order to improve subjective/objective videoquality.

FIG. 12 schematically shows a decoding apparatus for performing an imagedecoding method according to this document. The method disclosed in FIG.11 may be performed by the decoding apparatus disclosed in FIG. 12 .Specifically, for example, the entropy decoder of the decoding apparatusof FIG. 12 may perform S1100 of FIG. 11 ; the DPB of the decodingapparatus of FIG. 12 may perform S1110 to S1120 of FIG. 11 ; and thepredictor of the decoding apparatus of FIG. 12 may perform S1130 toS1140 of FIG. 11 .

According to this document described above, it is possible to signal theDPB parameter for the OLS, and through this, the DPB can be updatedadaptively to the OLS, and overall coding efficiency can be improved.

In addition, according to this document, index information indicatingthe DPB parameter for the OLS can be signaled, and through this, the DPBparameter can be derived adaptively to the OLS, and the overall codingefficiency can be improved by updating the DPB for OLS based on thederived DPB parameter.

In the above-described embodiment, the methods are described based onthe flowchart having a series of steps or blocks. The present disclosureis not limited to the order of the above steps or blocks. Some steps orblocks may occur simultaneously or in a different order from other stepsor blocks as described above. Further, those skilled in the art willunderstand that the steps shown in the above flowchart are notexclusive, that further steps may be included, or that one or more stepsin the flowchart may be deleted without affecting the scope of thepresent disclosure.

The embodiments described in this specification may be performed bybeing implemented on a processor, a microprocessor, a controller or achip. For example, the functional units shown in each drawing may beperformed by being implemented on a computer, a processor, amicroprocessor, a controller or a chip. In this case, information forimplementation (e.g., information on instructions) or algorithm may bestored in a digital storage medium.

In addition, the decoding apparatus and the encoding apparatus to whichthe present disclosure is applied may be included in a multimediabroadcasting transmission/reception apparatus, a mobile communicationterminal, a home cinema video apparatus, a digital cinema videoapparatus, a surveillance camera, a video chatting apparatus, areal-time communication apparatus such as video communication, a mobilestreaming apparatus, a storage medium, a camcorder, a VoD serviceproviding apparatus, an Over the top (OTT) video apparatus, an Internetstreaming service providing apparatus, a three-dimensional (3D) videoapparatus, a teleconference video apparatus, a transportation userequipment (e.g., vehicle user equipment, an airplane user equipment, aship user equipment, etc.) and a medical video apparatus and may be usedto process video signals and data signals. For example, the Over the top(OTT) video apparatus may include a game console, a blue-ray player, aninternet access TV, a home theater system, a smart phone, a tablet PC, aDigital Video Recorder (DVR), and the like.

Furthermore, the processing method to which the present disclosure isapplied may be produced in the form of a program that is to be executedby a computer and may be stored in a computer-readable recording medium.Multimedia data having a data structure according to the presentdisclosure may also be stored in computer-readable recording media. Thecomputer-readable recording media include all types of storage devicesin which data readable by a computer system is stored. Thecomputer-readable recording media may include a BD, a Universal SerialBus (USB), ROM, PROM, EPROM, EEPROM, RAM, CD-ROM, a magnetic tape, afloppy disk, and an optical data storage device, for example.Furthermore, the computer-readable recording media includes mediaimplemented in the form of carrier waves (e.g., transmission through theInternet). In addition, a bit stream generated by the encoding methodmay be stored in a computer-readable recording medium or may betransmitted over wired/wireless communication networks.

In addition, the embodiments of the present disclosure may beimplemented with a computer program product according to program codes,and the program codes may be performed in a computer by the embodimentsof the present disclosure. The program codes may be stored on a carrierwhich is readable by a computer.

FIG. 13 illustrates a structural diagram of a contents streaming systemto which the present disclosure is applied.

The content streaming system to which the embodiment(s) of the presentdisclosure is applied may largely include an encoding server, astreaming server, a web server, a media storage, a user device, and amultimedia input device.

The encoding server compresses content input from multimedia inputdevices such as a smartphone, a camera, a camcorder, etc. Into digitaldata to generate a bitstream and transmit the bitstream to the streamingserver. As another example, when the multimedia input devices such assmartphones, cameras, camcorders, etc. directly generate a bitstream,the encoding server may be omitted.

The bitstream may be generated by an encoding method or a bitstreamgenerating method to which the embodiment(s) of the present disclosureis applied, and the streaming server may temporarily store the bitstreamin the process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user devicebased on a user's request through the web server, and the web serverserves as a medium for informing the user of a service. When the userrequests a desired service from the web server, the web server deliversit to a streaming server, and the streaming server transmits multimediadata to the user. In this case, the content streaming system may includea separate control server. In this case, the control server serves tocontrol a command/response between devices in the content streamingsystem.

The streaming server may receive content from a media storage and/or anencoding server. For example, when the content is received from theencoding server, the content may be received in real time. In this case,in order to provide a smooth streaming service, the streaming server maystore the bitstream for a predetermined time.

Examples of the user device may include a mobile phone, a smartphone, alaptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), navigation, a slatePC, tablet PCs, ultrabooks, wearable devices (ex. Smartwatches, smartglasses, head mounted displays), digital TVs, desktops computer, digitalsignage, and the like. Each server in the content streaming system maybe operated as a distributed server, in which case data received fromeach server may be distributed.

The claims described in the present disclosure may be combined invarious ways. For example, the technical features of the method claimsof the present disclosure may be combined to be implemented as anapparatus, and the technical features of the apparatus claims of thepresent disclosure may be combined to be implemented as a method. Inaddition, the technical features of the method claim of the presentdisclosure and the technical features of the apparatus claim may becombined to be implemented as an apparatus, and the technical featuresof the method claim of the present disclosure and the technical featuresof the apparatus claim may be combined to be implemented as a method.

What is claimed is:
 1. An image decoding method performed by a decodingapparatus, the method comprising: obtaining image information includingDecoded Picture Buffer (DPB) parameter syntax structures and an OutputLayer Set (OLS) DPB parameter index for a target OLS; deriving DPBparameter syntax structure for the target OLS based on the OLS DPBparameter index; performing a picture management process for pictures ofa DPB based on the DPB parameter syntax structure for the target OLS;constructing a reference picture list for a current picture based on thepictures of the DPB; and performing inter prediction for a block in thecurrent picture based on the reference picture list, wherein the OLS DPBparameter index specifies the DPB parameter syntax structure for thetarget OLS among the DPB parameter syntax structures, wherein an OLS DPBparameter flag for whether the DPB parameter syntax structure for thetarget OLS is present in a Video Parameter Set (VPS) syntax is obtained,wherein the OLS DPB parameter syntax structure is obtained in the VPSsyntax based on a value of the OLS DPB parameter flag.
 2. The method ofclaim 1, wherein the OLS DPB parameter flag is included in the VPSsyntax.
 3. The method of claim 1, wherein the performing the interprediction for the block includes: deriving a reference picture in thereference picture list based on an obtained reference picture index forthe block; and deriving a prediction sample by performing the interprediction for the block based on the reference picture.
 4. The methodof claim 1, wherein the DPB parameter syntax structure for the targetOLS includes information on DPB size, information on maximum picturereorder number of the DPB for the target OLS and information on maximumlatency of the DPB.
 5. An image encoding method performed by an encodingapparatus, the method comprising: generating Decoded Picture Buffer(DPB) parameter syntax structures including DPB parameter syntaxstructure of a target Output Layer Set (OLS); performing a picturemanagement process for pictures of a DPB based on the DPB parametersyntax structure of the target OLS; constructing a reference picturelist for a current picture based on the pictures of the DPB; performinginter prediction for a block in the current picture based on thereference picture list; generating an OLS DPB parameter index for theDPB parameter syntax structure of the target OLS; and encoding imageinformation including the OLS DPB parameter index, the DPB parametersyntax structures and prediction information for the block, wherein theOLS DPB parameter index specifies the DPB parameter syntax structure forthe target OLS among the DPB parameter syntax structures, wherein an OLSDPB parameter flag for whether the DPB parameter syntax structure forthe target OLS is present in a Video Parameter Set (VPS) syntax isencoded, wherein the OLS DPB parameter syntax structure is signaled inthe VPS syntax based on a value of the OLS DPB parameter flag.
 6. Themethod of claim 5, wherein the DPB parameter syntax structure for thetarget OLS includes information on DPB size, information on maximumpicture reorder number of the DPB for the target OLS, and information onmaximum latency of the DPB.
 7. A non-transitory computer-readablestorage medium storing a bitstream including the image informationencoded by the image encoding method of claim
 5. 8. A transmissionmethod of data for image, the method comprising: obtaining a bitstreamof image information including Decoded Picture Buffer (DPB) parametersyntax structures and an Output Layer Set (OLS) DPB parameter index fora target OLS; and transmitting the data including the bitstream of theimage information including the DPB parameter syntax structures and theOLS DPB parameter index, wherein DPB parameter syntax structure for thetarget OLS is derived based on the OLS DPB parameter index, and apicture management process for pictures of a DPB is performed based onthe DPB parameter syntax structure for the target OLS, wherein the OLSDPB parameter index specifies the DPB parameter syntax structure for thetarget OLS among the DPB parameter syntax structures, wherein an OLS DPBparameter flag for whether the DPB parameter syntax structure for thetarget OLS is present in a Video Parameter Set (VPS) syntax is obtained,wherein the OLS DPB parameter syntax structure is obtained in the VPSsyntax based on a value of the OLS DPB parameter flag.